The effect of ternary additions of Group IIIb-IVbmetallic elements (Al,Ga,In, Tl,Pb) to Nb2Sn on the upper critical field Hc2(4.2K) has been studied. Samples were prepared by arc melting in an argon atmosphere. The measurements of Hc2(4.2K) were performed with a pulsed magnet, and the value of Hc2was determined from measurements of the electrical resistivity. Hc2(4.2K) of the Nb3Sn compounds was observed to increase by about 70 KG to a maximum of 300 KG through the replacement of several percent of the Sn by Group IIIb-IVbmetallic elements. Of these systems, Nb3(Sn-In) and Nb3(Sn-Pb) were also prepared by the diffusion method: Cold-rolled tapes of Nb doped with the third element were immersed and heat treated in a Sn bath or a Cu-30wt%Sn bath. Hc2as high as 270 KG was achieved for the Nb3(Sn-In) tape prepared in the Cu-30wt% Sn bath at a reaction temperature of 850°C. The critical current of this tape was substantially higher than that of the standard Nb3Sn tape in a high field region.