Photodynamic therapy based on photoactivable porphyrins (PAPs) can treat various dermatological conditions. The side-effects as well as the non-selective or insufficient accumulation of PAPs in the targeted tissues limit performances. We studied the stability in solution at different temperatures (21 degrees C; 4 degrees C), different pH values (7.5; 2.0), and as a function of time of 5-aminolevulinic acid's Tyrosine-ester, a molecule presenting interesting properties to selectively produce PAPs in blood vessels after topical application. Solutions of this precursor can be kept up to 24 h at refrigerated temperatures and under acidic pH. At room temperature or physiological pH, they must be prepared minutes before their use.