Experimental therapies for Barrett's esophagus, such as 5-aminolevulinic acid (ALA)–based photodynamic therapy (PDT), aim to ablate the premalignant Barrett's epithelium. However, the reproducibility of the effects should be improved to optimize treatment. Accurate irradiation with light of a proper wavelength (633 nm), fluence and fluence rate has shown to be critical for successful ALA-PDT. Here, we have used in situ light dosimetry to adjust the fluence rate measured within the esophagus for individual animals and monitored protoporphyrin IX (PpIX) fluorescence photobleaching simultaneously. Rats were administered 200 mg kg^–1 ALA (n = 14) or served as control (n = 7). Animals were irradiated with an in situ measured fluence rate of 75 mW cm^–2 and a fluence of 54 J cm^–2. However, this more accurate method of light dosimetry did not decrease the variation in tissue response. Large differences were also observed in the dynamics of PpIX fluorescence photobleaching in animals that received the same measured illumination parameters. We found that higher PpIX fluorescence photobleaching rates corresponded with more epithelial damage, whereas lower rates corresponded with no response. A two-phased decay in PpIX fluorescence could be identified in the response group, with a rapid initial phase followed by a slower rate of photobleaching. Nonresponders did not show the rapid initial decay and had a significantly lower rate of photobleaching during the second phase of the decay (P = 0.012).