A method for the treatment of a hydrocarbon well comprising administering down the well polymeric particles impregnated with a well treatment chemical or precursor or generator thereof. The particles have a pore volume of at least 20%, and are prepared by a process which comprises preparing an aqueous dispersion of polymer particles containing from 0.05 to 10 times by volume, based on the polymer, of one or more specific first material(s) adding a partly water-soluble second material having a water-solubility of at least ten times that of first material(s) under conditions which prevent or hinder transport of first material(s) through the aqueous phase, whereby second material diffuses into the polymer particles swelled with first material(s) and increases the volume of said particles by from 20 to 1000 times, based on the polymer.