Abstract.Valuations on a commutative ring, as defined by Manis, are considered in the special case where the domain of the valuation mapping is a ring of quotients of a given ring R. We consider relations between valuation mappings on various rings of quotients of a given ring. It is also shown that if K is any von Neumann regular ring of quotients of R, then any pair of nonassociates of R can be separated by valuations on K if and only if these elements are nonassociates in the integral closure of R in K.