Martensitic phase transformations in the solution-treated and water-quenched binary Ti-Nb alloys in the range of 16-26 at % Nb, were examined. An ordered, base-centred orthorhombic martensite was observed for alloys containing up to 23.4 at % Nb. The substructure of this martensite was generally composed of twins and stacking faults, the presence of antiphase boundaries observed in the plates indicating that the martensite underwent ordering during quenching. Both order-disorder and NJ,temperatures were. observed to be affected by total interstitial content, higher contents increasing both temperatures. Increasing the niobium content to above 23.4% resulted in retention of the beta phase, this phase containing either athermal omega or "diffuse" omega depending upon niobium and total interstitial concentration. Finally, the microhardness of the Ti-Nb alloys examined was observed to decrease with increase in niobium and decrease in total interstitial content.