The estimation is carried out by acetylation of the essential oil by acetyl chloride and acetic anhydride, in the presence of dimethylaniline. The acetylated oil is isolated and dried and the ester value is determined. The free alcohols content is calculated taking into account the ester values of the same but non-acetylated essential oil. This method is applicable to essential oils containing an appreciable proportion of tertiary alcohols, namely of linalol and terpineol. It is not applicable to essential oils containing appreciable proportions of phenols, anthranilates, lactones and aldehydes.