AbstractThis study deals with the effects of alcohols and ketones, two substances usually found in commercial zinc dialkyldithio‐phosphate (ZDDP), on the tribological behaviour of ZDDP. Experimental results show that two alcohol and ketone compounds, namely 2‐ethylhexanol and methyl isopropyl ketone, respectively, have an influence on the lubricity, wear resistance, and friction‐reducing properties of ZDDP. 2‐Ethylhexanol can increase the extreme‐pressure properties of ZDDP (the load‐carrying capacity of a lubricating oil being enhanced from 710 to over 1100 N). At the same time, the alcohol can also improve the wear resistance of ZDDP. However, this alcohol is detrimental to the antifriction properties of ZDDP, the friction coefficient of a lubricating oil being increased from 0. 0919 to 0. 1010. The antioxidative ability of ZDDP is weakened to some extent. The presence of a ketone, with a carbonyl group, greatly decreases the load‐carrying capacity of ZDDP, which goes down from 710 N to 597. 8 N on the addition of 1. 5 wt. % methyl isopropyl ketone to ZDDP. The mechanisms of the influence of the alcohol and ketone on ZDDP are also investigated.