Greenhouse studies on control of saltcedar (Tamarix pentandra Pall.), a problem phreatophyte, showed that when one of three hygroscopic compounds, polypropylenediol (a mixture of polypropylenediols with molecular weights ranging from 375 to 425) was added to water solutions of phenoxy herbicides, it significantly increased injury from most treatments. Injury to foliage from all herbicides tested, except the oil-soluble amine formulations (a mixture containing 80% tertiary dodecyl and 20% tertiary tetradecyl amine) of 2,4-dichlorophenoxyacetic acid (2,4-D), 2-(2,4,5-trichlorophenoxy) propionic acid (silvex), and 2,4-D plus 2,4,5-trichlorophenoxy acetic acid (2,4,5-T) (1:1), increased when polypropylenediol was added to the spray solution at 5% by volume. The increased activity was affected by the rate of the additive, temperature, humidity, and herbicide formulation.