In the present study, one hundred pure and adulterated honey samples obtained from feeding honeybee colonies with different levels (5,20 and 100 L/colony) of various commercial sugar syrups including High Fructose Corn Syrup 85 (HFCS-85), High Fructose Corn Syrup 55 (HFCS-55), Bee Feeding Syrup (BFS), Glucose Monohydrate Sugar (GMS) and Sucrose Sugar (SS) were evaluated in terms of the delta C-13 value of honey and its protein, difference between the delta C-13 value of protein and honey (Delta delta C-13), and C-4% sugar ratio. Sugar type, sugar level and the sugar type * sugar level interaction were found to be significant (P < 0.001) regarding the evaluated characteristics. Adulterations could not be detected in the 5 L/colony syrup level of all sugar types when the delta C-13 value of honey, Delta delta C-13 (protein-honey), and C-4% sugar ratio were used as criteria according to the AOAC standards. However, it was possible to detect the adulteration by using the same criteria in the honeys taken from the 20 and 100 L/colony of HFCS-85 and the 100 L/colony of HFCS-55. Adulteration at low syrup level (20 1/colony) was more easily detected when the fructose content of HFCS syrup increased. As a result, the official methods (AOAC, 978.17, 1995; AOAC, 991.41, 1995; AOAC 998.12, 2005) and Internal Standard Carbon Isotope Ratio Analysis could not efficiently detect the indirect adulteration of honey obtained by feeding the bee colonies with the syrups produced from C-3 plants such as sugar beet (Beta vulgaris) and wheat (Triticium vulgare). For this reason, it is strongly needed to develop novel methods and standards that can detect the presence and the level of indirect adulterations. (C) 2014 Elsevier Ltd. All rights reserved.