Two open reading frames in the genome of Sulfolobus solfataricus (SSO2342 [corrected] and SSO2424) were cloned and expressed in E. coli. The protein products were purified and their enzymatic activity characterized. Although SSO2342 [corrected] was annotated as a gene (gpT-1) encoding a 6-oxopurine phosphoribosyltransferase (PRTase), the protein product turned out to be a PRTase highly specific for adenine and we suggest that the reading frame should be renamed apT. The other reading frame SSO2424 (gpT-2) proved to be a true 6-oxopurine PRTase active with hypoxanthine, xanthine and guanine as substrates, and we suggest that the gene should be renamed gpT. Both enzymes exhibited unusual profiles of activity versus pH. The adenine PRTase showed the highest activity at pH 7.5-8.5, but had a distinct peak of activity also at pH 4.5. The 6-oxo PRTase showed maximal activity with hypoxanthine and guanine around pH 4.5, while maximal activity with xanthine was observed at pH 7.5. We discuss likely reasons why SSO2342 [corrected] in S. solfataricus and similar open reading frames in other Crenarchaeota could not be identified as genes encoding APRTase.