The heat flux measurements presented in this report were made during expedition SO240 on the German research vessel RV Sonne. The cruise started on the 3rd of may 2015 in Manzanillo (Mexico) and ended on the 16th of June 2015 in Manzanillo (Mexico). A total of 109 heat flux measurements were successfully carried out accompanied by detailed sediment echosounder and single channel seismic measurements on all heat flux profiles. The aim of the measurements was to investigate whether low-temperature hydrothermal fluids diffuse at the seafloor where the oceanic basement is not covered by sediments. While the measurements clearly showed that cold seawater enters the subsurface at one seamount investigated in detail, no diffuse leakage of hydrothermal fluids could be detected at any location.