The present invention relates to solid forms of 13-(N-Boc-ss-isobutylserinyl)-14-ss-hydroxybaccatin III 1,14-carbonate (Ortataxel). Amorphous Form A, crystalline Form B, mixtures thereof and processes for preparing them are disclosed. Amorphous Form A is prepared by fast precipitation of Ortataxel from a mixture of acetone and water. Form A transforms in Form B when suspended and stirred in a mixture of ethanol and water for 4-8 hours. If the suspension is stirred for less than 4 hours, mixtures of Form B and Form A are obtained. Form B or mixtures of Forms A and B can also be obtained dissolving Ortataxel in a protic organic solvent, followed by addition of water.