The yields of intermediate molecular weight products (PM = C7 to C11 = middle molecular weight product) have been determined for the radiolysis of 2-methylpentane and 3-methylpentane. A GC/MS system was used for the products identification. The product distribution determined by capillary column gas chromatography (GC2) reflects a marked structural effect in the radiolysis processes. In the solid radiolysis, the results suggest that the PM products do not originate from the combination of fragment and parent radicals. A chemical activation transfer to light olefinic products enclosed in the solid during the radiolysis plays a prominent role in the formation of the PM products.