The acylium perchlorate groups were successfully introduced onto the surface of a carbon whisker, i.e. a vapor-grown carbon fiber, by the reaction of acyl chloride groups on the surface with silver perchlorate in nitrobenzene at room temperature. The cationic ring-opening polymerizations of 1,3,5-trioxane and 1,3-dioxolane were initiated by the acylium perchlorate groups on the surface and the corresponding polyacetals were grafted onto carbon whisker: the percentage of poly(oxymethylene) grafting increased to 60%. The initiating activity increased with an increase in acylium perchlorate group content. Although the grafted poly(oxymethylene) on the surface was subject to stepwise thermal depolymerization from the chain end, the thermal stability was improved by acetylation of the end group. In addition, the acylium perchlorate groups on the surface have an ability to initiate the cationic copolymerization of 1,3,5-trioxane with styrene, styrene oxide, and 1,3-dioxolane. These polyacetal-grafted carbon whiskers produced stable dispersions in N,N-dimethylformamide.