Lamotrigine is an antiepileptic and mood stabilizing drug that has been detected in wastewater, groundwater, surface water and drinking water, at frequencies in surface water ranging from 47 to 97%. Because lamotrigine is a weak base (pKa = 5.7) that appears in two protonation states in natural waters, this study examined the direct photodegradation of lamotrigine (11.4 to 12.0 mg L(-1)) in simulated sunlight using liquid chromatography-UV diode array detection and buffered aqueous solutions at pH 3.3, 5.3, and 7.7. Lamotrigine's half-life varied little (100 ± 3 to 112 ± 2 h) with solution pH, but its specific light absorption rate was 12 times higher, and its reaction quantum yield was 13 times lower, at pH 7.7 versus pH 3.3. In the estimated midday, midsummer sunlight in Denver, CO, USA (latitude 39.8617 °N), lamotrigine's estimated photodegradation rate was more than twice as fast at pH 7.7 versus pH 3.3. Lamotrigine's photoproducts were detected by liquid chromatography-UV diode array detection and time-of-flight mass spectrometry. Solution pH was shown to affect the identities and relative abundances of lamotrigine's photoproducts. Some photoproducts appeared only in solutions containing protonated lamotrigine, and others appeared only in solutions containing neutral lamotrigine. As a result, different reaction mechanisms were proposed. Finally, lamotrigine's reaction quantum yield (2.51 ± 0.07 × 10(-5) mol einstein(-1) at pH 7.7) and other results suggested that lamotrigine and three photoproducts are approximately as resistant to direct photodegradation as carbamazepine, a frequently detected pharmaceutical in surface waters.