Mass transfer in a laboratory dyeing machine, consisting of cylindrical canisters mounted on a carousel with a horizontal axis, has been examined to adapt the unit for modification of cotton by admicellar polymerization. Canisters located at the periphery of the carousel are canted relative to the axis of rotation so that fluid in the canister moves front-to-back and back-to-front over the course of one revolution in a fashion similar to a rocking reactor. Dissolution of benzoic acid in water at 30 degrees C has been used to determine mass transfer coefficients beta in the dyeing system as a function of rotational rate and fill volume. Mixing in the dyeing system improved with canister fill fraction to an optimum of 0.9, while rotation rate had little effect over the available range. Findings for beta as well as predictions for beta obtained from estimates of specific mixing power are compared to values from the literature for a rocking reactor. The process of admicellar polymerization on cotton was subsequently examined in the laboratory dyeing machine. Cotton with a water repellent coating prepared in the unit was evaluated using various techniques. Conditions favorable to mass transfer in the model experiments were consistent with those providing optimal performance of the water-repellent coating applied by admicellar polymerization.