In situ probing of liquid-solid interfaces is important for understanding heterogeneous liquid-phase catalysis and other interfacial phenomena, but the spectroscopic interference from the bulk is often a problem. Some organics may have infrared features overlapping the adsorbed CO peaks, making the determination of adsorbed CO difficult. In this study, CCl4-flushing was used as a novel diagnosing tool to pin down the low-coverage CO derived from decarbonylation of organics. This diagnosing tool was designed based on our in situ reflection-absorption infrared spectroscopy results reported here that there is a marked solvent effect (water > ethanol > methanol > cyclohexane > benzene similar to carbon tetrachloride) on CO oxidation at the liquid-solid interface. Possible reasons for that solvent effect were discussed. (c) 2006 Elsevier Inc. All rights reserved.