The effect of micro-oxygenation on the aromas of two varietal wines (Tempranillo and Cabernet Sauvignon) when alcoholic fermentation is complete was examined. Several factors, such as whether malolactic fermentation (MLF) took place in barrels or stainless steel vats and ageing time in oak barrels, were taken into account. Major aroma components were studied using GC-FID and minor compounds were studied by GC–MS analysis before and after MLF and after 4 and 8 months of maturation in oak barrels. Sensorial analysis was performed to describe and quantify the different wine notes. Micro-oxygenation affects different compounds but the effect depends to a great extent on the grape varietal used; the effect of micro-oxygenation was more marked on the aroma of Cabernet Sauvignon wine than Tempranillo wines. The concentration of extractable compounds of wood was higher in wines after MLF in barrels than in wines after MLF in steel vats; wines with steel vat MLF had fewer toast and wood notes. The compounds that showed significant differences in concentration after 8 months of maturation were present in higher concentrations in wines fermented in barrels than in stainless steel vats. Barrel-fermented wines were sweeter, with wood and toast notes, fewer alcohol notes and fewer reductive notes.