Seniority isomers are nuclear states with an electromagnetic decay that is hindered by selection rules related to the seniority quantum number. A simple analysis is presented of their possible formation with reference to the nickel isotopes 70−76Ni and the N = 50 isotones from molybdenum to cadmium. It is shown that the existence of seniority isomers in a j = 9/2 shell is predominantly governed by the quadrupole pairing matrix element of the nucleon-nucleon interaction. The analysis is generalized to shells with larger j.