1. Fluorescence changes of umbelliferone have been used as indicator for pH changes in the outer phase of the functional membrane of photosynthesis. In combination with the repetitive pulse technique it is possible to measure the pH change which is produced during one turn-over of the molecular machinery (Δ pH ≈ 10−4). The analysis of this primary pH changes gives insight in the primary ionic events in general. 2. The rise of the primary pH formation takes place in 20-30 ms. The decay is at 20°C of first order with k ≈ 0.7 s−1 or τ½ ≈ 1 s. 3. The rise of the pH formation corresponds to the decay of the electrical field across the membrane (pHout 7, pHin 7, isotonic solution). 4. The amplitude of the primary pH change indicates the ratio of the field driven K⊕ and H⊕ efflux across the membrane **. This has been proved by membrane active substances which are specific for K⊕ and H⊕ permeabilities respectively. 5. The ratio of the field driven K⊕ and H⊕ efflux depends on the pHin. At pHin 7 (pHout 7) the field drives 100% K⊕. With decreasing pHin the efflux shifts from K⊕ to H⊕ (at pHin 5, pHout 7 50% K⊕ and 50% H⊕). 6. The field driven K⊕ efflux causes the formation of the pH changes. The field driven H⊕ efflux causes no pH changes. 7. The K⊕ channel and the H⊕ channel and its regulation by pHin lead to an extended concept of the coupling of the primary events in photosynthesis. This is discussed in chapter II and fig. 4.