The dynamics of bacterial growth and concentrations of oxygen and carbon dioxide in the headspace of modified atmosphere packaged (MAP) meat is studied. The work is based on extensive long-time storage experiments on pork chops lasting up to 42 days at temperatures 0 degrees C and 5 degrees C and four different headspace gas mixtures with 10%, 20%, 40% and 70% oxygen and 30% carbon dioxide. The headspace dynamics is modelled. The model includes oxygen consuming biochemical processes in the meat and also the coupling between headspace and meat, specifically, the diffusion of oxygen and carbon dioxide into the meat. The model is parametrized from the experimental data, and captures the experimental results. It is concluded that the oxygen consuming processes inside the meat packed with MAP consume less than 1% of the initial oxygen in the headspace, and are not important for the headspace dynamics. On the other hand, the dissolution of carbon dioxide has a significant effect. This is due to the relatively large solubility of carbon dioxide compared to oxygen. From the model we find and characterize three distinct phases behind the headspace volume reduction observed. As the model is dynamical and mechanistic it can be used to give information about the retail-packed fresh meat under different oxygen containing atmospheres, effects of leaks, and more.