1. THE PROCESS OF MODIFYING SOLID LINEAR FIBER-FORMING POLYESTERS OF TEREPHTHALIC ACID AND A GLYCOL HAVING FROM 2 TO 10 CARBON ATOMS WHICH COMPRISES GENERATING CARBONYL GROUPS IN THE SURFACE OF THE POLYESTER BY EXPOSING THE SAME TO OXIDIZING CONDITIONS WITHOUT SIGNIFICANT DEGENERATION OF THE POLYESTER AND THEREAFTER CONTACTING WITHOUT SIGNIFICANTLY DISSOLVING THE ACTIVATED SURFACE WITH A DILUTE SOLUTION OF A HYDRAZINE COMPOUND OF THE FORMULA R1R2NNH2 IN WHICH EACH R1 AND R2 IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF HYDROGEN MONOVALENT HYDROCARBON RADICALS, HETEROHYDROCARBON RADICALS, NITRO SUBSTITUTED HYDROCARBON AMINO SUBSTITUTED HYDROCARBON HYDROXY SUBSTITUTED HYDROCARBON, HALOGEN SUBSTITUTED HYDROCARBON, AND HETEROHYDROCARBON RADICALS, UNTIL A CHANGE IN THE COLOR OF THE POLYESTER TAKES PLACE.