The origin of heat-induced chlorophyll fluorescence rise that appears at about 55–60°C during linear heating of leaves, chloroplasts or thylakoids (especially with a reduced content of grana thylakoids) was studied. This fluorescence rise was earlier attributed to photosystem I (PSI) emission. Our data show that the fluorescence rise originates from chlorophyll a (Chl a ) molecules released from chlorophyll-containing protein complexes denaturing at 55–60°C. This conclusion results mainly from Chl a fluorescence lifetime measurements with barley leaves of different Chl a content and absorption and emission spectra measurements with barley leaves preheated to selected temperatures. These data, supported by measurements of liposomes with different Chl a /lipid ratios, suggest that the released Chl a is dissolved in lipids of thylakoid membranes and that with increasing Chl a content in the lipid phase, the released Chl a tends to form low-fluorescing aggregates. This is probably the reason for the suppressed fluorescence rise at 55–60°C and the decreasing fluorescence course at 60–75°C, which are observable during linear heating of plant material with a high Chl a /lipid ratio ( e.g. green leaves, grana thylakoids, isolated PSII particles).