Isobaric vapor-liquid equilibria for both 1-octene+propenoic acid and 2-butyl propenoate+propenoic acid systems have been determined at 6.67 kPa in a small-capacity recirculating still. Results were found to be thermodynamically consistent by using the L-W method of Wisniak and they were satisfactorily correlated with the van Laar, Wilson, and UNIQUAC equations, considering propenoic acid as an associating compound in both liquid and vapor phases.