The collision of axisymmetric turbulent plumes with buoyancy fluxes of opposite sign is examined experimentally. The total buoyancy flux loss of each plume as a result of the collision is measured. The measurements are made using a new experimental technique for measuring the buoyancy flux of a plume based on the ventilation theory of Linden, Lane-Serff & Smeed (J. Fluid Mech. vol. 212, 1990, p. 309). The experimental results are presented as functions of the buoyancy flux ratio $\psi$ and the ratio of radial to vertical separation $\sigma $. For axially aligned plumes we find that the lower-buoyancy-flux plume loses all its buoyancy flux when $\psi \,{<}\,$0.3, and that there is very little buoyancy flux loss for either plume when $\sigma \,{>}\,$0.25. This plume–plume collision is modelled using a modified set of entrainment equations. The model allows for the exchange of buoyancy and deflection of the plumes as they pass by each other. We present predictions of total buoyancy flux loss as a function of both plume strength and separation. The model predictions are compared to the experimental measurements of buoyancy flux loss, and show good agreement.