The reaction of atomic thorium cations with CH4 (CD4) and the collision-induced dissociation (CID) of ThCH4+ with Xe are studied using guided ion beam tandem mass spectrometry. In the methane reactions at low energies, ThCH2+ (ThCD2+) is the only product; however, the energy dependence of the cross-section is inconsistent with a barrierless exothermic reaction as previously assumed on the basis of ion cyclotron resonance mass spectrometry results. The dominant product at higher energies is ThH+ (ThD+), with ThCH3+ (ThCD3+) having a similar threshold energy. The latter product subsequently decomposes at still higher energies to ThCH+ (ThCD+). CID of ThCH4+ yields atomic Th+ as the exclusive product. The cross-sections of all product ions are modeled to provide 0 K bond dissociation energies (in eV) of D-0(Th+H) = 2.25 +/- 0.18, D-0(Th+CH) = 6.19 +/- 0.16, D-0(Th+CH2) = 4.54 +/- 0.09, D0(Th+CH3) = 2.60 +/- 0.30, and D0(Th+CH4) = 0.47 +/- 0.05. Quantum chemical calculations at several levels of theory are used to explore the potential energy surfaces for activation of methane by Th+, and the effects of spin-orbit coupling are carefully considered. When spinorbit coupling is explicitly considered, a barrier for C-H bond activation that is consistent with the threshold measured for ThCH2+ formation (0.17 +/- 0.02 eV) is found at all levels of theory, whereas this barrier is observed only at the BHLYP and CCSD(T) levels otherwise. The observation that the CID of the ThCH4+ complex produces Th+ as the only product with a threshold of 0.47 eV indicates that this species has a Th+(CH4) structure, which is also consistent with a barrier for CH bond activation. This barrier is thought to exist as a result of the mixed (F-4,D-2) electronic character of the Th+ J = (3)/(2) ground level combined with extensive spinorbit effects.