Biodiesel is derived from vegetable oils and animal fats by various methods among which transesterification is the most widely used. The present work explores the effects of variations in free fatty acids (FFA) of vegetable oils on the biodiesel production methodology as well as on the yield. Three vegetable oils, viz. corn, neem and rubber seed, having FFAs of 0.28, 12.1 and 47.1%, respectively, are mixed in various proportions to get 19 oil samples with FFAs varying from 0.28 to 47.1%. An alkali transesterification method is used for oil samples having FFAs ranging from 0.28 to 5%. The biodiesel yield is around 95% for oils having FFA below 2%; however, it drops below 80% for oils with FFA above 3.6%, while the process fails for oils with FFA above 4.2%. A multi-stage acid–base transesterification process is employed for oils having high FFA, above 6%. A single-stage acid pretreatment process is found to be enough for oils with FFA up to 24%, and a multi-step pretreatment is required thereafter. The measured viscosity, density and calorific value of all the biodiesels are found to be well within the international standard specifications. © 2017 Informa UK Limited, trading as Taylor & Francis Group