The preparation of two peanut butter reference materials and the certification of their aflatoxins B1,B2,G1,G2and total aflatoxin contents is described. The materials were prepared and certified within the BCR Programme of the Commission of the European Community as part of a broad activity to improve accuracy and agreement of measurements of importance in food and agriculture (Wagstaffe and Belliardo 1990). Reference material RM 385 was prepared from naturally contaminated peanuts, roasted and ground into a paste and then blended with uncontaminated peanut butter to achieve the desired aflatoxin concentrations. Details are given of the blending and canning procedure, and the checks to ensure homogeneity and stability of the material. Reference material RM 401 was similarly prepared but from an uncontaminated peanut butter. The certification exercise was carried out by nine laboratories using a variety of extraction and clean‐up procedures, but all using high performance liquid chromatography (HPLC) as the determinative stage although operating under a variety of chromatographic conditions. RM 385 was certified as containing aflatoxins B1, B2, G1and G2at levels of 7.0 ± 0.8μg/kg, 1.1 ± 0.2μg/kg, 1.7 ± 0.3μg/kg and 0.3 ± 0.2μg/kg respectively (total aflatoxin content of 10.1 ± 1.5μg/kg) and RM 401 as containing aflatoxin B1, B2and G2 at <0.2μg/kg and aflatoxin G1at <0.3μg/kg (total aflatoxin content <0.9μg/kg). The materials are intended for the verification of methods used to determine aflatoxins in nuts and nut products.