By the application of zero-field magnetic resonance (ZFMR)/FID technique and fast-field sweeping around the level anti-crossing region with fixed rf frequency, we obtained the (minimum) energy gaps between the two crossing levels as 16.7 +/- 2.0 and 14.3 +/- 3.7 MHz for pentacene-h(14) in p-terphenyl and pentacene-d(14) in p-terphenyl, respectively, when B(0)parallel tox. The minimum energy gaps occur at 14 mT for both systems. The hyperfine interaction contributes only partially to the observed energy gap.