The entrainment of a turbulent jet in coflow, with and without reaction, is directly measured using Particle Image Velocimetry (PIV). By measuring the velocity at the outside of the main jet, the entrainment measurement does not suffer from density fluctuations that exist in reacting jets. The direct measurement technique is verified by measuring the entrainment of a nearly free jet, and comparing it with the results of previous measurements. Both averaged and instantaneous velocity fields surrounding jets are observed and the effects of coflow speed, heat release, and buoyancy on the entrainment rate are investigated. The increase in coflow speed reduces the entrainment for both reacting and nonreacting jets. Heat release suppresses entrainment by a factor of approximately 2.5 (for a typical hydrocarbon fuel in air) when compared to the nonreacting jet, but buoyancy recovers the entrainment to a faster rate. Therefore, for the initial part of a reacting jet where the jet is momentum driven, heat release has the dominant effect on entrainment, but buoyancy eventually compensates the reduction further down-stream. The entrainment rate of a lifted flame in coflow is measured as a further example of the experimental method. As a practical application, the NOx emission of hydrogen flames in different coflow speeds is predicted with the Two-Stage Lagrangian (TSL) model by using the measured entrainment rate of reacting jets in coflow. (C) 2001 by The Combustion Institute.