Fuel change over method for an internal combustion engine provided with a system for supplying a first fuel and at least a second fuel, two or more storage tanks for the first fuel and the second fuel, each storage tank having a respective switch valve associated thereto that is capable of selectively managing the sending of said first fuel and said second fuel, the method comprising the steps of: - calculation of the target temperature value that the second fuel or the fuel blend of said first fuel and said second fuel should have in the step of introduction into the internal combustion engine; - variation ( δT) of the temperature of the second fuel, or the fuel blend of said first fuel and said second fuel, in the step of introduction into the internal combustion engine, to reach the target temperature; - calculation of the time ( t) needed to reach the target temperature; - calculation of the viscosity ( υ F 1( t)) of the first fuel and of the viscosity ( υ F 2( t)) of the second fuel at a given temperature as a function of the time; and - calculation of the percentage (% F 1( t)) of the first fuel and of the percentage (% F 2( t)) of the second fuel in the fuel blend of said first fuel and said second fuel needed at each instant to ensure the viscosity ( υ ( t)) which said fuel blend should have in the step of introduction into the internal combustion engine, so that the viscosity variation or gradient ( δυ) of said fuel blend introduced into the internal combustion engine remains within predefined limit values during the entire fuel change over procedure through the control, performed by said switching valves, of the composition of said fuel blend that is generated during said change over procedure, the method being characterised in that it comprises the step of: - recalculation, according to the volume of the supply system and of the instantaneous consumption ( C ( t)) of the internal combustion engine, of the percentage (% F 1 In ( t)) of the first fuel and of the percentage (% F 2 In ( t)) of the second fuel to be introduced in the supply system, wherein said volume of the supply system is defined as a parameter. - calculation of the target temperature value that the second fuel or the fuel blend of said first fuel and said second fuel should have in the step of introduction into the internal combustion engine; - variation ( δT) of the temperature of the second fuel, or the fuel blend of said first fuel and said second fuel, in the step of introduction into the internal combustion engine, to reach the target temperature; - calculation of the time ( t) needed to reach the target temperature; - calculation of the viscosity ( υ F 1( t)) of the first fuel and of the viscosity ( υ F 2( t)) of the second fuel at a given temperature as a function of the time; and - calculation of the percentage (% F 1( t)) of the first fuel and of the percentage (% F 2( t)) of the second fuel in the fuel blend of said first fuel and said second fuel needed at each instant to ensure the viscosity ( υ ( t)) which said fuel blend should have in the step of introduction into the internal combustion engine, so that the viscosity variation or gradient ( δυ) of said fuel blend introduced into the internal combustion engine remains within predefined limit values during the entire fuel change over procedure through the control, performed by said switching valves, of the composition of said fuel blend that is generated during said change over procedure, - recalculation, according to the volume of the supply system and of the instantaneous consumption ( C ( t)) of the internal combustion engine, of the percentage (% F 1 In ( t)) of the first fuel and of the percentage (% F 2 In ( t)) of the second fuel to be introduced in the supply system, wherein said volume of the supply system is defined as a parameter.