Fate of organic contaminants under UV irradiation as environmental variable was studied. Aqueous solutions of pentachlorophenol and phenanthrene were subjected to simulated solar radiation and monitored during ≤93 h. Immobilization tests were conducted using Daphnia magna neonates in pre-irradiated water for the assessment of photoproduct-mediated toxicity. We observed a time-dependent decrease in exposure concentrations following irradiation, which correlated positively with reduced immobilization of the animals. A complete disappearance of the lowest and highest concentrations of pentachlorophenol was noted after 25 h and 75 h, respectively. Survivorship of the animals increased until 100% and correlated positively with irradiation time. However, phenanthrene was rather persistent under irradiation, with less than 25% decline in exposure concentrations after 93 h. Neonates were not immobilized at maximum aqueous solubility of phenanthrene. Rate constants ( k) for the photodegradation of pentachlorophenol at 0.41, 0.59, 1.1, and 2.1 mg l −1 were in the range of 7.2 × 10 −2 and 4.9 × 10 −2 h −1, showing a slight decrease with increasing initial pentachlorophenol concentration. Nonetheless, pentachlorophenol degradation in the studied concentration range could still be described by the pseudo-first-order kinetics. K values for phenanthrene at 0.12 and 0.22 mg l −1 were 2.9 × 10 −3 h −1 and 4.2 × 10 −3 h −1, respectively.