Gas chromatography-olfactometry is a commonly used technique for analysis of odour active compounds. The present study deals with the influence of methodological factors on gas chromatography-olfactometry data, i.e. assessors' qualities and analytical conditions. With respect to the assessors' qualities, the effects of training and fatigue/alertness were studied. Analytical conditions included concentration aspects resulting from varying chromatographic peak shapes and humidified air flow rates at the sniff port. The compounds 2-butanone, diacetyl, ethyl acetate, 3-methyl-1-butanol, ethyl butyrate, hexanal, 2-heptanone and α-pinene were analysed by gas chromatography-olfactometry using the detection frequency and posterior intensity method for collection and processing of the data. Training did not affect the detection of the odour active compounds, but it decreased noise levels considerably. No significant effect of fatigue or decreased alertness was found for the 45-min sessions. The data were influenced by extreme peak shape differences and an intensity optimum was observed for all compounds at a humidified air flow rate of 6 ml min–1. Assessors' qualities and analytical conditions were shown to affect GC-O data and require optimisation for sound gas chromatography-olfactometry analysis.