The effects of structure, concentration and composition of host-odour plumes on catch of female Anopheles gambiae Giles sensu stricto and Aedes aegypti (L.) (Diptera: Culicidae) were investigated in a dual-choice olfactometer. We demonstrate that the fine-scale structure of host-odour plumes modulates capture of An. gambiae and Ae. aegypti. In both species homogeneous skin-odour plumes result in trap entry, whereas homogeneous CO2 plumes reduce trap catch. Reduced trap catch also result from combining skin odour with a homogeneous CO2 plume. Trap capture rates in homogeneous CO2 plumes were concentration-dependent and differed between the two species. Electric nets placed in front of the trap entrances intercepted mosquitoes before they could enter the traps. This showed that An. gambiae flew along CO2 plumes, but did not enter the traps. Survivorship analysis of the trap-entry times of Ae. aegypti indicated interactions between the time until capture and treatment. The assay's duration therefore can alter the distribution in a dual-choice olfactometer.