The composting treatment and the application on soil of the obtained compost are, in the current context, some interesting recovering ways for organic wastes. The effectiveness of a composting treatment and its potential negative impacts depend on the preparation of initial wastes and on the process management. The waste initial characteristics which are essential for the course of a composting treatment concern the amount and type of biodegradable organic matter, the nutrients ratio, etc but also the physical structure. Indeed, this latter will influence both the substrate biodegradability and the aeration conditions. In this context, the aim of this work was to study the influence of the initial preparation of mixtures of sludge and bulking agent on its initial physical characteristics (free air space FAS, air permeability K, specific surface Sv), and on its biodegradability and its subsequent behaviour during composting. Three operational parameters for waste physical preparation were considered: bulking agent particle size, mixture ratio between sludge and bulking agent, moisture content of the mixture. The obtained results were correlated to the three operational parameters. This work was a part of a parent project named ESPACE which was carried out in partnership with the Suez Environment Group and IMFT (a French Public Research Centre), and jointly financed by the ANR (the French National Research Agency). An agro-food wastewater treatment sludge and two bulking agents (BA1 green wastes from a commercial sludge composting plant, BA2 shredded wood pallet) were used. For each of these two bulking agents, mixtures were prepared (based on experimental design) with five sizes of particles (range 8mm to more than 40mm), five moisture contents (range 20-70%) and five dry mass ratios of sludge/BA mixture (range 1:2.8 to 1:9.2). Initial physical characterization of mixtures was carried out using an experimental apparatus allowing the study of 40-50L batches and combining features of an air pycnometer with those of an air permeameter. The initial specific surface was estimated by using the Kozeny-Carman relation. The waste biodegradability was determined on 10L samples by respirometric tests under controlled aeration, moisture and temperature. The composting behaviour was studied using 300L laboratory insulated composters. A statistical analysis of the results was led in order to determine the significant effects and interactions of the three operational parameters on the considered responses: initial FAS, K and Sv, total oxygen consumed during respirometric or composting trials, total biodegradable organic matter, biomass growth rate, material consolidation during composting, airflow dispersion coefficient in composting reactor. For all responses and whatever the bulking agent was, the initial moisture content appeared to be the most influent operational parameter, with (i) a global positive effect on oxygen uptake during respirometric tests or composting treatment, biodegradable organic matter, biomass growth rate, material consolidation and (ii) a global negative effect on initial FAS, K, Sv and airflow dispersion coefficient (D'). The bulking agent particle size had a significant positive effect on initial K and D', and a negative impact on initial Sv, oxygen uptake, biodegradable matter, growth rate and consolidation. The mixture ratio seemed to have a low impact on initial physical characteristics but had to be taken into account for substrate biodegradability. The significant trends showed that compromises were necessary for operational parameters in order to favour both biodegradability and composting efficiency without reaching limitations in initial physical structure. Some physical limitations were observed during composting experiments conducted at very high initial moisture content, with an oxygen uptake decrease in these conditions. Very low initial air permeability could explain the observed physical limitations and, in the case of recycled green wastes used as bulking agent, initial free air space below 30% also contributed to these physical limitations. The results of this study helped to understand the influence of the physical preparation of wastes on the effectiveness of the subsequent composting treatment.