An enzyme is here described in rat liver cytoplasmic extracts which hydrolyzes dinucleoside triphosphates. Compounds of this kind are present in Artemia salina and Daphnia magna extracts and capping the 5′ end of RNAs of diverse origin and nature. Diadenosine, diguanosine and diuridine triphosphates were substrates of the reaction with similar V and Km values (7, 2 and 25 μM respectively). The products of the reaction were the corresponding nucleoside di and monophosphates. Dixanthosine triphosphate was the unique dinucleoside triphosphate tested which was not substrate of the enzyme. None of the following compounds were substrates of the reaction: dinucleoside tetraphosphates (diadenosine, and diguanosine tetraphosphates), dinucleoside diphosphates (diadenosine and diguanosine diphosphates, NAD+, NADP+), nucleoside 5′-phosphates (AMP, ADP, ATP, p4A), bis-p-nitrophenyl phosphate and glucose 6-phosphate. The enzyme requires Mg2+ or Mn2+, is maximally active at a pH value of approximately 7.5, and has a molecular weight of 29800 as estimated by filtration on Sephadex G-75. AMP, ADP and ATP were competitive inhibitors of the reaction with Ki values in the 100 μM range. Based on its substrate specificity the name of dinucleosidetriphosphatase or dinucleoside-triphosphate nucleotidehydrolase has been adopted for the enzyme through this paper. This enzyme is different from dinucleosidetetraphosphatase (EC 3.6.1.17) and from other enzymes recently described which liberate m7pG or m7GDP from the dinucleoside triphosphate caps of certain RNAs.