Cyanide ion in water is converted to cyanate ion by use of macroporous poly(styrene-co-divinylbenzene) bearing regenerable N-chlorinated N-alkylsulfonamide functional groups. Cyanide is converted first to cyanogen chloride that is then rapidly hydrolyzed to cyanate ion, which is stable at high pH. Cyanate ion can be further oxidized by active chlorine supplied by the resin to nitrogen and carbonates under weakly basic conditions or more rapidly hydrolyzed to ammonium ion and carbon dioxide by passage through a bed of a strong acid cation-exchange resin in the H+ form without use of additional active chlorine. Much of the active chlorine that escapes the reactor can be recaptured by unchlorinated polymer for subsequent use.