Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)-aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide-metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a (Cp2ThH3AlCTMS3)-Th-tt [TMS = Si(CH3)(3); Cp-tt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp2TiH3AlCTMS3 (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, H-1 and Al-27, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (a(iso) = -9.7 and -10.7 MHz, respectively), the spin density on the Al ion in the Th3+ complex is similar to 5-fold larger than that in the titanium(3+) (Ti3+) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.