Determining the diastereoselectivity of new synthetic mo­lecules can be a challenge when NMR methods fail and the compounds are difficult to crystallize. Encapsulating organic crystals can be used to overcome this challenge. Here we show that the diastereomeric confi­guration of racemic mixtures of propargyl cyclopropanes can be determined by co-crystallization with 1,3,5,7-tetrakis(2-bromo-4-methoxyphenyl)adamantane and X-ray crystallography. Three crystal structures are reported that unambiguously identify the products of Fe-catalyzed cyclopropanations as cis - or trans -isomers. These findings expand the scope of co-crystallization with tetraaryladamantanes as a method to determine the stereochemical configuration of organic molecules that are difficult to crystallize by themselves.