Rhodopseudomonas capsulata chromatophores were immobilized with a co-crosslinking method. Immobilization was used as a tool for a defined modification of the chromatophore environment to study ATP production over a long period of time. The light-induced phosphorylation of ADP as a function of time was studied with chromatophores under different conditions: (a) native chromatophores with and without the hexokinase ATP-trapping system; (b) immobilized chromatophores without hexokinase, with the enzyme added in the bulk solution and with the enzyme co-immobilized in the matrix. The overall amount of ATP produced as a function of ADP concentration was studied for native and immobilized chromatophores. The global phosphorylation performed was also studied as a function of the amount of biological material used. The results can be explained by an effect of the ATP/ADP ratio. The results given by the immobilization show that the important point is not the ATP/ADP ratio in the bulk solution but the ratio value in the microenvironment of the chromatophore itself.