Technical Abstract: A previous study has revealed a positive correlation between heat-induced damage to photosynthetic membranes (thylakoid membranes) and chlorophyll loss. In this study, we exploited this correlation and developed a model for prediction of thermal damage to thylakoids. Prediction is based on estimation of the ratio of constant chlorophyll a fluorescence (O) and the peak of variable fluorescence (P) (O/P). The model was developed using 12 cultivars of hexaploid winter wheat (Triticum aestivum L.) Cultivars were exposed to heat stress for 16 d, and during the treatment chlorophyll a fluorescence and chlorophyll content were measured at two-day intervals. Chlorophyll content was measured using a SPAD chlorophyll meter. The relationship between O/P and chlorophyll content was determined using random coefficients regression analysis and a simple linear regression equation was developed for prediction of thylakoid damage. The model was tested in six genotypes of hexaploid wheat, 25 genotypes of tetraploid wheat (T. turgidum L.), and 20 genotypes of maize (Zea mays L.). Predictive ability was assessed by analyzing the relationship between the predicted and measured O/P. The model adequately predicted O/P and thereby the heat stability of thylakoid membranes in all genotype groups with high coefficients of determination (r2 > 0.80). The model could be used as an easy and inexpensive means for detection of the structural and functional state of photosynthetic membranes in wheat and maize, and possibly other crops, in hot environments.