bitumen / polymer compositions are produced by contacting , between 100 ° c . and 230 ° c ., under agitation , one bitumen or a mixture of bitumens with at least one olefinic polymer bearing epoxy or cooh groupings , then by incorporating in the product , an acid additive while the whole mixture is agitated at a range between 100 ° c . and 230 ° c . for at least 10 minutes . the product , which is subjected to treatment by the acid additive may still contain one or several additional polymers , for instance of the elastomer type cross - linkable with sulphur , and even be subjected to sulphur cross - linking before treatment . the bitumen / polymer compositions are useful directly or after dilution , to form binders for carrying out surfacing operations .