The work described in this report was undertaken, within the context of the 12 Tesla program, to investigate the possibility of increasing the upper critical field, Hc2, of Nb-Ti through alloying additions. A preliminary report has previously been given by us. In the first part of this paper we report further measurements of Hc2in the Nb-Ti-Ta and Nb-Ti-Hf systems. Whilst we find only small enhacements of ∼0.3 Tesla in μoHc2(4.2K) compared to binary Nb-Ti, at 2K there is a wide composition range in the Nb-Ti-Ta system where μoHc2(2K) exceeds 15 Tesla, reaching a maximum of 15.5 Tesla. This represents an enhancements of 1.3 Tesla over unalloyed Nb-Ti. By comparison alloys in the Nb-Ti-Hf system show a maximum enhancement in μoHc2(2K) of only 0.3 Tesla. The reasons both for the enhancements in Hc2and for the differences in behavior systems shown by alloys containing Ta and Hf are briefly discussed. In part II we discuss common features in the behavior of the high field critical current density, Jc, of four commercial Nb-Ti composites and upon the basis of this behavior predict the enhancements in the high field Jcto be expected from using Nb-Ti-Ta and its alloys.