Abstract: A method of isomerizing ?9-tetrahydrocannabinol (“?9-THC”) to ?10-tetrahydrocannabinol (“?10-THC”). The method includes the steps of: extracting ?9-THC from cannabis biomass, which optionally contains one or more of the components found in fire retardant such as PHOS-CHEK®; dewaxing of crude extracts by winterization; pH-adjusting extracts by washing the extracts in heptane solution with aqueous solutions of: citric acid, sodium bicarbonate, and brine; isomerizing ?9-THC to ?10-THC by exposure to suitable conditions and in the presence of a catalyst based on the components of fire retardant; vacuum distillation of ?10-THC at a predetermined temperature range and vacuum level; collecting the distillate and redistilling it up to three times to acquire distillate containing less than 60% ?10-THC; and purification of the MO-THC to a purity of 99% or greater by crystallization from n-pentane solution.