The respiration rate (RR) of mushroom (Agaricus bisporus Lange, strain X25) under air follows Arrhenius’ law with an apparent activation energy of 43 400 J mol−1 corresponding to a Q10 of 2.9 between 10 and 20°C. Oxygen from 20 to 1 kPa did not affect RR at temperatures ranging from 5 to 20°C, confirming previous studies. The initial RR of mushroom (same cultivar, flush, growing conditions) measured 24 h after harvesting can vary from 1 to 2 mmol O2 kg−1 h−1 at 10°C. The apparent Km value was below 0.1 kPa at 10°C, and could not be estimated more accurately due to instrumental limitations. Decrease in dry matter of modified atmosphere-packed mushrooms matched the theoretical carbon consumption calculated from the constant respiration rate. Glucose and glycogen were rapidly catabolized, whereas mannitol consumption began 3 and 8 days after harvest at 20 and 10°C, respectively. It was concluded that no extension of mushroom shelf life was attainable through modified atmosphere packaging. Controlling RH within the package is likely to be more effective.