Abstract.1. This paper describes field observations on diel changes in the nectars ofCrataegus, TiliaandEchiumin relation to microclimate and insect visits.2. Nectar concentration is highly correlated with ambient relative humidity, but the concentrative properties of the nectars differ from those of pure sugar solutions in ways that could be accounted for by microclimate or chemical effects.3. The microclimate inside flowers may influence the rate of equilibration of nectar with the relative humidity of the air, or the equilibrium concentration itself.4. The vapour pressure relationships of nectars may be influenced by the presence of components with lowr/Pvalues (that is, solutes in which solution show a relatively large lowering of vapour pressure,P, for a small change in refraction,r). Such components might be released by micro‐organisms in the nectar.5. The quantity of sugar per flower depends on the relative rates of secretion and reabsorption, among other things. The flowers we studied showed evidence for morning and evening peaks of secretion, and inCrataegussubstantial quantities of sugar were ‘sequestered’ around midday.6. The pattern of visits by bumblebees to the flowers ofTiliaandEchiumcan be related to the changing concentration of sugar in the nectar; caloric reward was probably not limiting for bumblebees visiting