Acetal of warfarin acid has been synthesized via a hemi acetal intermediate and structurally characterized by X-ray diffraction. The crystal structure shows that the title compound was formed by the hvdrolysis of warfarin acid in methanol at pH 3, with space group P2(1)/n and unit cell parameters a = 5.859 (17): b = 16.745 (5), c = 16.402 (5) angstrom, alpha = 90.00; beta = 94.853 (4), and gamma = 90.00 degrees.