To provide a dual functional cellulosic additive which comprises a specific cellulose ether, provides associative thickening and rheological properties to a latex compsn. during storage and application when compounded into the compsn., and can promote the crosslinking of the latex film to provide a hard and durable coating after the compsn. is applied to the surface to be coated.

This crosslinkable additive has a hydrophobic substituent having a 4-20C alkyl portion having an unsaturation (pref. polyunsaturated) enough to promote the crosslinking between additive molecules on exposure to oxygen and comprises a cellulose ether substd. by 0.001-0.1 mol (per mol thereof) of the hydrophobic substituent.