The effect of light and thermally initiated E-Z isomerization on the anion sensing of two easy synthesized and efficient isatin N-phenylsemicarbazone E-isomer colorimetric sensors Ia and IIa for strongly basic anions were investigated. The photodegradation of E-isomer anion sensors due to light initiated E-Z isomerization during anion sensing may complicate the anion detection, therefore care should be taken when interpreting data for quantitative determination of anions using sensors with the tendency towards E-Z isomerization. However, the photochemical E-Z isomerization efficiency for Ia and IIa is relatively low at Ф_E-Z < 0.01, and this allows reliable detection of strongly basic anions. Moreover, despite of lower Z-isomer sensitivity, the utilization of both E- and Z-isomer enlarges the detection range for F^- or CH₃COO^- anions valid for Ia and IIa in organic media from approximately 1×10^-6-1×10^-4 mol dm^-3 to 1×10^-6-1×10^-2 mol dm^-3 of F^- or CH₃COO^-. Zero efficiency of both photochemically and thermally initiated back Z-E isomerizations was observed.¹