The effects of heat treatment on the formation of chlorophyll (Chl) derivatives and Chl-degrading enzyme activities in stored broccoli (Brassica oleracea L. Italica Group ‘Ryokutei’) florets were determined. The Chl a level barely changed during heat treatment, but each Chl a derivative level changed. Chlorophyllide (Chlide) a, pheophorbide (Pheide) a, C13²-hydroxychlorophyll (C13²-OHChl) a, Chl a′, an isomer of Chl a, and pheophytin (Phy) a were detected as a Chl derivative during heat treatment and during storage after treatment. Chlide a, Pheide a, and C13²-OHChl a levels decreased during 2-h heat treatment (50°C), whereas Chl a′ and Phy a levels increased. Chl-degrading enzyme activities, in particular, Mg-dechelation activity, were effectively suppressed after 2 h of heat treatment. The content of Chls a and b in control broccoli florets decreased greatly during storage at 15°C, while the content in heat-treated broccoli florets hardly changed at all. In Chl derivatives, the Pheide a level in broccoli florets treated with or without heat treatment, especially the former, increased appreciably during storage. The Chlide a level in control florets decreased markedly during storage. On the other hand, the Chlide a level in heat-treated broccoli florets did not change during storage. Mg-dechelation activity in control florets markedly increased after 4 days of storage at 15°C, but the enhancement of the activity was suppressed by heat treatment. These findings suggest that Chl derivatives, especially Pheide a, are accumulated as intermediates in heat-treated broccoli florets, and Mg-dechelating action, in conjunction with that of chlorophyllase and Chl-degrading peroxidase, could be involved in Chl degradation in stored broccoli florets.