In this letter we show that the use of slightly broadband laser sources like multimode alexandrite lasers for CH PLIF bears the danger of interferences from formaldehyde. Such interferences occur when the laser frequency is not carefully tuned to the B-X (0,0) R-branch band head at around 387.2 nm and manifest either in the appearance of an additional layer in the PLIF image or in a slight broadening of the actual CH layer. With careful frequency selection the formaldehyde signals were not observed. Suppression of the interferences is feasible by employing appropriate spectral filters. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.