We report high-resolution and high-precision measurement of hyperfine interactions of the first excited electronic state (B) of 12 over an extensive range of vibrational and rotational quantum numbers. Systematic variations in the hyperfine parameters, including both rovibrational level dependence and internuclear separation dependence, are confirmed by calculations based on ab initio molecular potential energy curves and electronic wave functions derived from a separated-atom basis set. We have accurately determined the state-dependent quantitative changes of hyperfine interactions caused by perturbations from other electronic states and identified the respective perturbing states. (C) 2003 Published by Elsevier B.V.