The aim of the paper is to analyse the factors that determine Spanish consumers' intention to purchase organic food. Then, an intention to purchase model based on the Theory of Planned Action (Ajzen) has been specified taking into account also other factors such as organic food knowledge and consumers' concerns on health and environment. An ordered bivariate Probit model has been estimated with information from a personal survey conducted to consumers in two Spanish regions. Results indicate that consumers' knowledge about organic food followed by consumers' attitudes towards the purchase and the individuals perceived behavioural control are the main factors determining consumers' intention to purchase organic food. However, neither social norms nor consumers' concerns on health and environment affect the intention to purchase organic food. A segment of potential consumers of organic food products have been detected and the size of the segment could be increased if consumers' knowledge about organic food increases. Finally, organic knowledge is positively related to the income and education level and to the consumers' environmental consciousness.