The ability of positron emission profiling (PEP) to measure concentration profiles of molecules labelled with positron-emitting nuclei, such as C-11, N-13, and O-15, inside chemical reactors has been demonstrated for the system n-hexane-Pt/H-zeolites under conditions typical of the hydroisomerization reaction. Data obtained in the absence of reaction were first measured and used to model mass transport processes in these biporous, packed-bed reactors. Images obtained under conditions where injected, labelled pulses underwent reaction revealed that the products did not exit the reactor and thus demonstrated the need for in situ measurement. Such experiments should provide a valuable new tool in the study of transient, initial phenomena so often of importance in heterogeneous catalysis, such as "preconditioning" and deactivation.