Surface effects on AgCu nanoparticles synthesized in aqueous solution of 1,10-phenathroline were investigated by Knudsen effusion mass spectrometry during slow heating in order to understand their thermal stability and behaviour. Selection of a mass range for mass spectra monitoring was performed on the basis of data acquired by direct inlet probe mass spectrometry during rapid heating realized prior the Knudsen effusion method. As follows from the measurements, the organic substance which protects the nanoparticles against coagulation suddenly and quantitatively evaporates at elevated temperatures during the heating process. The data were subsequently used for specification of surface effects on the nanoalloys. Results are summarized and discussed in view of previous studies.