The effects of solute type and concentration upon the nature of, and stresses for, plastic flow in polycrystalline iron solid solutions have been examined. An extensive study was made using Al, Be, Ge, P, Si, and Sn; limited examples with Co, Sb, and Ti are also included. Three experimental observations stand out: (i) At room temperature and lower solute concentration the flow stress for slip increases monotonically with the parameter c|ln km|. Here c is the solute concentration, and the solid–liquid distribution coefficient, km, differentiates the solute type, (ii) At room temperature, there is a transition in the deformation mode from slip to twinning at a concentration that decreases with increasing |ln km|. The actual stresses for mechanical twinning can be compared with those at 77 °K; they are higher and provide a second measure for the deformation-mode transition, (iii) At 77 °K the initial deformation mode is predominantly mechanical twinning, and, below concentrations producing atomic ordering, the initial flow stresses are independent of solute concentration and type.When these observations are accompanied by tests for the propagation-controlled phenomenon of continual mechanical twinning, we deduce the following: (i) and (ii) show that solute promotes mechanical twinning as a consequence of its effect on dislocation behavior, most likely by requiring fast-moving individual dislocations, while (iii) shows that a threshold stress exists for the nucleation of mechanical twins that cannot easily be explained by existing twin-nucleation mechanisms.