A technique has been developed enabling oil samples to be taken continuously from the piston ring grooves and related areas of engines during typical operation. A detailed study of oil samples obtained from this critical region of two single-cylinder IDI diesel engines has been undertaken utilising sophisticated micro-analytical techniques which allow a time resolution of 3 h or better. Analysis has shown major differences between these oil samples and oil samples from the sump. Oil degradation (as indicated by analysis) in the ring zone is extensive even in the first samples obtained during a run, with oxidative attack and base oil evaporation being the dominant effects. Subsequently the oil sample analyses show only slow changes as a quasi steady-state is reached. Differences between these samples and the sump oil persist, with the sump oil showing very little sign of degradation even after extended running.