AbstractThe intention of this paper is to examine the relationship between the chemical structure of ester base fluids and their influence on elastomer seals and wear characteristics. It presents background information about esters, some of which can be considered as blending components for mineral oil and/or polyalphaolefins, while others have properties so similar to mineral oil that in principle substitution is possible. It examines the effects of ester base fluids on a number of properties of elastomers, including modulus, tensile strength, elongation at break, hardness, and volume changes. It also explains a method of selecting synthetic base fluids for performance upgrading or viscosity purposes, using a ‘non‐polarity index’ which allows more speedy comparison and selection of ester base f