The relationship between the spraying volume and glyphosate effectiveness has been widely studied over the years. Likewise, the effect of different adjuvants on glyphosate leaf adherence/penetration is currently well understood. However, not much is known about the way spraying volumes may influence these two herbicide properties or, in other words, which factor (adherence or penetration) is the limiting factor when spraying at low or high volumes of treatment. Using grass-weed Lolium rigidum and broad-leaved weed Portulaca oleracea as experimental systems, the effect of low (200 L ha(-1)) and high (800 L ha(-1)) spraying volumes and six commercial adjuvants (poly-l-p-menthene, a mixture of methyl oleate and palmitate, alkylglycol ester, dodecylbenzene ammonium sulphonate, and two paraffinic oils) plus ammonium sulfate on the effectiveness of five commercial glyphosate formulations has been studied under laboratory controlled conditions. Dose-response assays showed significant differences between low and high spraying volume treatments as expected, the former being much more effective than the latter. However, and in all commercial formulations and weed species tested, the best results at low spraying volumes were always obtained when formulating glyphosate with oils or their derivatives, with a minimal improvement observed in glyphosate/surfactant mixtures. On the contrary, both the use of ionic or non-ionic surfactants as glyphosate adjuvants gave the best results at high spraying volumes, with only a marginal success observed on paraffinic oils and fatty acidesters. Data point out glyphosate absorption as being a two-step process made up of adherence first and then penetration, both events being essential for glyphosate effectiveness. Under these terms, high performance glyphosate-rich low spraying volumes succeed by themselves in soaking leaf surfaces, penetration therefore being the main and only limiting factor, while low performance glyphosate-poor high spraying volumes fail to either soak or penetrate leaf cuticles. The use of penetrative agents such as oils is therefore advised as a single method to increase glyphosate effectiveness at low spraying volumes, whereas glyphosate treatments at high spraying volumes are better improved by using surfactant agents or surfactant + oil mixtures.