We present an in-flight fluorescence detection scheme for molecular beams which is applied to determine the enthalpy of sublimation of dye molecules. We investigate tetraphenylporphyrin (TPP), porphine, and nile red, which are believed to be suitable candidates for molecular de Broglie wave interferometry [L. Hackermuller , Phys. Rev. Lett. 91, 090408, 2003]. The measured values are H-sub(TPP)=142+/-3 kJ/mol, H-sub(porphine)=87+/-3 kJ/mol, and H-sub(nile red)=66+/-2 kJ/mol. For TPP, sublimation enthalpies differ in the literature by more than a factor of 2. Our measurements confirm a value at the lower end of this scale. We discuss changes in the character of the molecular flow with the source temperature as a prime reason for discrepancies in the published data. (C) 2004 American Institute of Physics.