Cooking meat products results in substantial mass loss. Much effort is expended on strategies to improve yield, by additives or by process modification. Producers are also concerned to minimise vapour emissions. Meat patties of 10 and 20 mm thickness were prepared by adding beef fat to lean beef so as to prepare five fat content ranges from 0% to 40% fat by weight. Each patty was cooked to > 70 degreesC at the core, using mid- and far infrared radiative heating, source temperatures 800 degreesC and 500 degreesC, respectively. By computer-aided continuous weighing during the cooking cycle it was possible to monitor vaporised and liquid mass losses independently. During cooking a beefburger patty loses mass partly by vaporisation of cellular water. Vaporisation of water in cooking meat is not a drying process. Very little fat is vaporised. Evaporation commenced within the first minute of the cooking process and, once established for either heat source, the rate remained the same for all fat contents and all surface temperatures. The rate of evaporative loss was limited by source energy. The rate was approximately 2.7 times greater with mid-infrared radiation. A liquid runoff collection system combined with analysis of the post-cooked product enabled liquid and vapour cookout to be segregated into water-based and fat fractions. Surface and interior temperatures were measured by thermocouples and recorded by a datalogger. Final surface temperature increased with fat content, from 105 degreesC for no fat added, to 240 degreesC for maximum fat content. The mass transfer mechanism was complex; it could be partly attributed to Fickian concentration driven molecular diffusion, partly by capillary force and partly by bulk movement under gravity. Some of the apparent mass transfer resulted from the assumption of cellular water by the retreat of the evaporation boundary. For the most part the diffusion portion of the mass transfer should be classified as non-Fickian, being characterised by the capillary-porous structure of ground and bowl-chopped beefburger and bulk moisture release throughout the body of material. In addition to moisture migration to the evaporation boundary, through it and removal to atmosphere as vapour, there was an excess of moisture created. This excess removed itself as drip, by bulk transfer under gravity, without passing through the evaporation boundary. Following an initial settling in period, the rate of vaporisation of moisture was independent of interior temperature or availability of moisture. Water-based cooking loss was mostly vaporised; fat-based cooking loss was almost exclusively in drip.