The genetically-encodable fluorescent tag miniSOG is expected to revolutionize correlative light- and electron microscopy due to its ability to produce singlet oxygen upon light irradiation. The quantum yield of this process was reported as ΦΔ = 0.47 ± 0.05, as derived from miniSOG's ability to photooxidize the fluorescent probe anthracene dipropionic acid (ADPA). In this paper, a significantly smaller value of ΦΔ = 0.03 ± 0.01 is obtained by two alternative and independent methods: direct measurement of its phosphorescence at 1275 nm and chemical trapping using uric acid as an alternative probe. We give further insight into the photochemistry of miniSOG and we ascertain the reasons for the discrepancy in ΦΔ values. Our findings show that miniSOG oxidizes ADPA by both singlet oxygen-dependent and -independent processes. In addition, we find that cumulative irradiation of miniSOG increases its ΦΔ value ca. 10-fold due to a photoinduced transformation of the protein. This may be reason why miniSOG outperforms any other fluorescent protein reported to date as singlet oxygen generator.