A test road constructed with municipal solid waste incineration (MSWI) bottom ash was monitored over a period of 36 months. Using chemical and toxicological characterisation, the environmental impact of leachates from bottom ash was evaluated and compared with leachates from gravel used as reference. Initial leaching of Cl, Cu, K, Na, NH4–N and TOC from bottom ash was of major concern. However, the quality of the bottom ash leachate approached that of the gravel leachate with time. Leachates from the two materials were compared regarding the concentration of pollutants using multivariate data analyses (MVDA). A standardized luminescent bacteria assay using Vibrio fischeri did not show any toxicity, most likely because saline contamination can mask the toxic response and stimulate luminescence in these marine bacteria. A mung bean assay using Phaseolus aureus revealed that the toxicity of bottom ash leachate collected at the very beginning of the experimental period (October 2001 and May 2002) might be attributed to the following components and their respective concentrations in mgl−1: Al (34.2–39.2), Cl (2914–16,446), Cu (0.48–1.92), K (197–847), Na (766–4180), NH4–N (1.80–8.47), total-N (12.0–18.5), and TOC (34.0–99.0). The P. aureus assay was judged as a promising environmental tool in assessing the toxicity of bottom ash leachate.