[en] The independent isomer yield ratio for ⁹⁰Rb from thermal neutron fission of ²³⁵U has been measured by use of a new technique involving a pulsed reactor and an on-line mass spectrometer facility. The apparent isomer yield ratio was measured for different ion collection time intervals and extrapolated to zero collection time to eliminate interference from ⁹⁰Kr decay. The observed isomer yield ratio of 8.7 +- 1.0 is one of the largest ratios measured for a low energy fission process. However, a statistical model analysis shows that the average angular momentum (< J> = 4.5) deduced from this isomer yield ratio is consistent with average angular momentum for other products from low energy fission. 7 refs