A process for making a crimped tow of filaments suitable for forming a bale of crimped tow for conversion to cigarette filter rods comprising, presenting an uncrimped tow of filaments in a banded condition, adhesively bonding to the filaments of the banded tow (3), porous particles having absorbent surfaces imparting filtration capacity for tobacco smoke constituents, and crimping the banded tow, wherein the process comprises the steps, of: (i) pre-treating the particles to load them with a material capable of generating a gaseous emission from the particles thus forming pre-treated particles; (ii) applying to the filaments of the banded tow, the pre-treated particles and an adhesive for bonding the particles to the filaments; and subsequently (iii) treating the tow to generate the gaseous emission from the pre-treated particles so as to limit deactivation of the absorbent particle surfaces by the adhesive.