Variations in the abundance of isotopes of elements in primitive meteorites carry the record of chemical and nuclear processes that occurred during the formation of the Solar System. Here I explore the possibility that photochemical self-shielding of carbon monoxide, a process that is known to occur in molecular clouds, may also have been important in the solar nebula. In the solar nebula, the process is based on far-ultraviolet radiation from the growing Sun, which is effective over a small distance in the inner part of the nebula. In order to acquire their observed isotope compositions, all of the solid matter in the present inner Solar System must have been processed through this region, and subsequently expelled to greater distances by an X-wind or similar mechanism¹.