The invention relates to a method for determining the heat flow (dQ/dt) emanating from a heat transporting fluid (12), which is a mixture of at least two different fluids, and which flows through a flow space (11) from a first position, where it has a first temperature (T1), to a second position, where it has, due to that heat flow (dQ/dt), a second temperature (T2), which is lower than said first temperature (T1), whereby the density and specific heat of said heat transporting fluid (12) is determined by measuring the speed of sound (vs) in said fluid, and said density and specific heat of said heat transporting fluid (12) is used to determine the heat flow (dQ/dt).