PURPOSE: To provide the titled compsn. which gives an excellent co-crosslinked product, consisting of a fluororubber, a halongen-contg. polymer or a diene rubber, a di- or trithiol-S-triazine compd., an onium salt and a metallic compd.

CONSTITUTION: 0.1W10tps.wt. di- or trithiol-S-triazine compd. of fomula I (wherein R is OR1, NR3R4; R1WR4 are each H, an org. residue; M is H or a mono- or bivalent metal), 0.1W10pts.wt. onium salt of formula II (wherein X is P, N; Y is halogen; at least one of R1WR4 is a 1W20C hydrocarbon group which may have a substituent and the other is a primary to tertiary amino group or a fluoroalkyl) and 0.1W20pts.wt. metallic compd. such as MgO, are blended with 100pts. wt. mixture of 1W99wt% fluororubber such as polytetrafluoroethylene (A) and 99W1wt% halogen-contg. polymer (e.g. chlorinated PE) other than component A or diene rubber (e.g. chloroprene) (B). The blend is kneaded at 80W250°C.