Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
substantiate [səbˈstænʃɪeɪt] vt (claim, story, statement etc) обосно́вывать (обоснова́ть perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: