Chlorophyll α fluorescence induction kinetics is commonly used as a probe of photosynthesis. Here, an ODE model developed by Stirbet et al. [J. Theor. Biol. 193 (1998) 131], describing the O–J–I–P fluorescence transient on the basis of the redox reactions occurring around photosystem II centres, is fitted using the software package PARFIT to the fluorescence induction data measured ‘in vivo’ on pea leaves. After a short review of the mathematical model of the fast chlorophyll α fluorescence induction process, the numerical results are presented. The approximations of the data are very good, but some model parameter values were not in agreement with those reported in the literature. In order to solve this problem we have also considered the presence of some non-Q B active photosystem II units (around 20%), and postulated an exponential law for plastoquinone (PQ) exchange reaction. Moreover, the number of the PQ pool molecules was considered as model parameter in the fitting procedure. The results obtained after these improvements are closer to the reported data.