Process for preparing high molecular weight polycarboxylic acid acylating agents by contacting chlorine with a reaction mixture comprising (1) high molecular weight monocarboxylic acid and ethylenically unsaturated low molecular weight mono- or polycarboxylic acid acylating agent or (2) high molecular weight aliphatic hydrocarbon, ethylenically unsaturated low molecular weight monocarboxylic acid acylating agent, and ethylenically unsaturated low molecular weight polycarboxylic acid acylating agent. Acylating agents thus produced have utility, for example, as intermediates in the preparation of acylated nitrogen compounds, esters, metal salts, and the like which are useful as additives in lubricants and fuels.