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