The diffusion length of thermal neutrons has been measured in water over the range 25.9 to 295.3°C in the pressure vessel of the KAPL High Temperature Critical Assembly. The diffusion length was determined by fitting an exponential to the data found by activating manganese foils with neutrons from a small Sb-Be source. The temperature variation of the diffusion length could be fairly well represented by either of two simple approximations—either that the transport cross section of water has a 1/υ behavior, or that the transport cross section is determined from the Radkowsky prescription.