Experiments with zeolite Beta and zeolite Y demonstrate that adamantane and methyl adamantanes can be isolated very effectively from modem refinery streams by mild hydrocracking over Pt- and Pd-containing large pore zeolites. Yield depends importantly on individual refinery crude source and process configuration. Heavy crudes and refineries with conventional hydrocracking and FCC feed hydrotreater facilities are particularly desirable, and an ideal feed for adamantane isolation in such a situation is the 150-250 degrees C fraction of the hydrocracker (HDC) recycle stream. When Pt- or Pd-containing zeolite Beta was used with such a stream, temperatures of some 250 degrees C and pressures below 3.5 mPa (500 psig) sufficed to remove selectively well over 90% of the non-adamantane hydrocarbon, with little conversion of adamantanes. High selectivity for adamantanes is attributed in large part to size-selective exclusion of these molecules from the pores of zeolite Beta.