Significance A major hurdle in understanding how phytochromes (Phy) regulate plant photomorphogenesis has been defining how the related isoforms work collectively and uniquely to regulate specific cellular activities. Here, we compared their biophysical properties using recombinant chromoproteins from several plant species. While phototransformation rates between the Pr and Pfr states were similar, strong isoform-specific differences in thermal reversion rates were evident, ranging from minutes to days. Notably, PhyB couples rapid thermal reversion with a strong temperature dependence on the rate to serve as the main isoform enabling thermoperception. While the properties of PhyA make it suited for measuring low-light environments, those for PhyB fit well with full sun environments, which together allow acclimation to an array of terrestrial environments.