In the late 1980s, operators of trunk-piston marine engines encountered the problem of contamination of engine lubricating oils by fuel. This paper summarises the results of two surveys conducted by CEC IL-047, with cooperation from members of the CIMAC Lubricants Working Group and the Marine Engineering Society of Japan. The surveys ascertained the frequency and severity of problems in trunk-piston engines that arose either from contamination of crankcase oil by raw fuel or from interactions between the oil and combustion products. It was found that although contamination was most prevalent in the camshaft housing and the crankcase, the greatest impact on engine operation came from deposits on the fuel-pump plunger and on piston undercrowns. In parallel, a survey and a critical review were conducted of currently available analytical methods used for detecting the contamination of lubricants by marine residual fuel. It appears that there is no standard simple, reliable, accurate and cheap method of detecting contamination of trunk-piston engine lubricants by fuel. The interpretation of results from existing macro-or micro-analytical methods is limited by the varied characteristics of the fuels in use, and by the changes that take place in both fuels and lubricants during service.