This article discusses the investigation of an incident caused by the mixing of incompatible chemicals: a strong oxidizing agent with a reducing agent. The exact mixture and sequence of mixing was unknown, but the materials included an aqueous solution of 40% sodium permanganate and either solid sodium thiosulfate or solid sodium metabisulfite. These chemicals were part of a new technology being pilot-tested for remediating contaminated groundwater. Throughout the pilot test, excess waste permanganate was transported by hand in 5-gallon pails from the injection area to a designated nearby area for reduction. During one such transfer, a worker was seriously burned when a pail of sodium permanganate solution erupted onto him. The liquid splashed onto and ignited his clothing. A puzzling feature of this accident was the apparent time delay between chemical addition and the subsequent reaction. Various chemical addition scenarios were evaluated by thermodynamic calculations, followed by laboratory, pilot, and full-scale tests. The test results revealed that while some chemical addition scenarios resulted in a prompt exothermic reaction with the generation of steam; other scenarios resulted in a similar but delayed reaction. The time delay was caused by mass transfer limitations associated with the solids dissolution process. This case study serves as a reminder that the hazardous result of mixing incompatible chemicals may be delayed and may occur when least expected. (C) 2011 American Institute of Chemical Engineers Process Saf Prog 30: 148-153, 2011