The behavior of different lysine Amadori compounds during acid hydrolysis was investigated in order to determine the molar yield of furosine and the other hydrolysis products. Based on this, the conversion factors for calculating, the content of Amadori compound and lysine modification before hydrolysis can be derived. For that purpose, the peptide-bound Amadori compounds N-epsilon-(1-deoxy-D-fructosyl)-, N-epsilon-(1-deoxy-D-tagatosyl)-, N-epsilon-(1-deoxy-D-lactulosyl)- and N-epsilon-(1-deoxy-D-maltulosyl)hippuryllysine as well as free N-epsilon-(1-deoxy-D-fructosyl)lysine were synthesized. For isolation of peptide-bound Amadori compounds, an optimized enzyme-enhanced reversed phase-high pressure liquid chromatography procedure was developed. Pyridosine and N-epsilon(carboxymethyl)hippuryllysine were synthesized as reference materials. After acid hydrolysis with 6 M hydrochloric acid the, molar yields of furosine were determined to be 32% for fructosyllysine, 34% for lactulosyl- and maltulosyllysine and 42% for tagatosyllysine. Hydrolysis with 8 M hydrochloric acid resulted in a higher yield of furosine for Amadori compounds containing a fructosyl-moiety, 46% for fructosyllysine, 50% for lactulosyllysine and 51% for maltulosyllysine. Compared with this, the molar yield for furosine was not affected by concentration of hydrochloric acid in the case of tagatosyllysine. Based on these conversion factors a reliable calculation of the amount of Amadori compound or lysine modification and with it the evaluation of the progress of the "early" Maillard reaction in foods and biological samples is possible via the quantification of lysine and furosine after acid hydrolysis.