Results are presented from permeameter tests involving unidirectional seepage flow through reconstituted specimens of four widely graded cohesionless soils. The onset of instability is defined by a significant decrease in local hydraulic gradient over a relatively short period of time. The novel concept of a hydromechanical path in stress (sigma'(vm)) - gradient (i(jk)) space is proposed, which describes the response to seepage flow during testing and terminates at the value of critical hydraulic gradient. The path terminus establishes a hydromechanical boundary governing the onset of seepage-induced internal instability in one-dimensional flow. The boundary represents a failure envelope, which is different for each of the four soils tested. A ranking of seepage-induced instability for each soil, from most unstable to least unstable, is found similar, but not identical to, the susceptibility to internal instability determined from empirical analysis of the gradation shape.