The present invention relates to polyol esters that are particularly suitable for use as insulating oils in electrical equipment where an effective cooling action is required, such as electrical transformers. In particular the invention relates to esters of trimethylolpropane with monocarboxylic acids of nine and/or ten carbon atoms, wherein the monocarboxylic acids of nine and/or ten carbon atoms comprise 5-15 mol % of branched acids and 85-95 mol % of linear acids with respect to the total moles of the monocarboxylic acids.