A stable, ready-to-use injectable solution including thiotepa and one or more pharmaceutically acceptable solvents, co-solvents, and/or solubilizing agents. Formulations retain at least 90% and as high as 99% of the purity of thiotepa as measured by HPLC after storage at 25° C./60% RH for a period of 7 days. Certain formulations retain at least 99% of the purity of thiotepa as measured by HPLC after storage for at least six months at 2-8° C.
