This thesis describes an investigation into the foam generation mechanisms involved in producing foam from a lox expansion fire fighting branchpipe. The investigation was carried out using scale models of branchpipes, and a high-speed video camera was used to study the formation of the foam. The experiments provided evidence of three possible methods of bubble formation : Stage 1 - Mixing within the branchpipe Stage 2 - Air entrainment and bubble growth during the flight of the jet Stage 2 - Aeration produced from the collision of the high speed jet onto a surface Each stage is described in detail and the mechanism which has the greatest effect on the expansion ratio of the foam produced has been determined. The relevance of these findings to the design of branchpipes is discussed.