Tracer studies employing 2,2-dichloropropionic acid (dalapon)-2-C14 and -Cl36 were conducted on barley (Hordeum vulgare L.), bean (Phaseolus vulgaris L.), and three hypostomatous species—zebrina (Zebrina pendula Schnizl.), coleus (Coleus blumei Benth.), and nasturtium (Trapoelum majus L.). Count data and radioautographic evidence showed that greater amounts of dalapon were absorbed and translocated at high (88 ± 3%) than medium (60 ± 5%) or low (28 ± 3%) post-treatment relative humidity (herein called R.H.). Also, greater uptake and translocation of C14 occurred in bean at 43 ± 1 C than at 26 ± 1 C post-treatment temperature, with constant R.H. and light conditions. Rate of droplet drying, and stomatal distribution and behavior, apparently contributed to the R.H. effect. Droplets dried less rapidly at high than at low R.H., thus prolonging the period of effective absorption. At low R.H., periodic rewetting of the droplet area enhanced absorption and translocation but never to the extent achieved with high ambient R.H. (in growth chambers) or in saturated atmosphere (polyethylene bagging in the greenhouse). High R.H. (95 ± 3%) favored stomatal opening in zebrina, and uptake of dalapon was greater through the abaxial than adaxial surface of leaf disks of zebrina, coleus, and nasturtium. Rewetting of droplets or high ambient R. H., however, promoted uptake through astomatous as well as stomated surfaces. Absorption and translocation were markedly greater in bean plants grown at 95 ± 3% R.H. than at 28 ± 3% R.H., both treated under high R.H. conditions. Thus cuticle hydration and ontogeny also were implicated in the R.H. phenomenon. Several solution additives also were studied in an attempt to substitute for the R.H. effect. At low R.H., a surfactant (X-77) markedly enhanced uptake of dalapon in bean, glycerol (a humectant) and hexadecanol (a filming agent) were less advantageous, and Plyac (a sticker-spreader) decreased penetration. Results of these studies aid in explaining the greater toxicity of dalapon in regions of high R.H. and, conversely, its sometimes erratic performance in drier climates. The results also confirm the distribution and accumulation patterns of dalapon absorbed by plants and the usefulness of an effective spray adjuvant in promoting foliar absorption.