A process for forming self-aligned, twin well regions for a CMOS device, without the use of an oxidation retarding silicon nitride layer, has been developed. A first ion implantation procedure is used to place N type ions in a first portion of a semiconductor substrate, followed by a wet thermal oxidation procedure resulting in the growth of a thick silicon dioxide layer on the N type ions, in the first portion of the semiconductor substrate, while growing a thin silicon dioxide layer on a second portion of the lightly doped, P type semiconductor substrate. A second ion implantation procedure places P type ions through the thin silicon dioxide layer, into the second portion of the semiconductor substrate, while the thick silicon dioxide layer prevents the P type ions from reaching the first portion of the semiconductor substrate. A subsequent anneal procedure results in the formation of a N well region, in the first portion of the semiconductor substrate, self-aligned to the formed P well region, located in the second portion of the semiconductor substrate.