Abstract: The invention relates to the production of silica reinforced rubber incorporating a novel azo-silane coupling agent which does not depend on the vulcanization reaction to effect coupling to the rubber. The novel coupling agents are of the general formula:Y -- X -- CO -- N .dbd. N -- CO -- X.sup.1 -- Zwhere X and X.sup.1 are the same or different and each is an imino group, an oxygen atom, or a substituted or unsubstituted methylene group; Y is a substituted or unsubstituted alkyl, aryl or aralkyl group or is the same as Z; and Z as an alkyl, aryl or aralkyl group which has an substituent at least one silane group of the formula: --Si(OR).sub.3 or --OSi(OR).sub.3 in which R is a straight or branched chain alkyl group. The azo-silane is typically used in amounts of from .1 to 10 pphr. Silica reinforced rubber vulcanizates produced using the azo-silane can have improved properties e.g. abrasion resistance, scorch resistance and resilience as compared with vulcanizates made using conventional coupling agents.