Monodisperse CoSn and NiSn nanoparticles were prepared in solution and supported on commercial carbon black. The obtained nanocomposites were applied as anodes for Li- and K-ion batteries. CoSn@C delivered stable average capacities of 850, 650, and 500 mAh g at 0.2, 1.0, and 2.0 A g, respectively, well above those of commercial graphite anodes. The capacity of NiSn@C retained up to 575 mAh g at a current of 1.0 A g over 200 continuous cycles. Up to 74.5 and 69.7% pseudocapacitance contributions for Li-ion batteries were measured for CoSn@C and NiSn@C, respectively, at 1.0 mV s. CoSn@C was further tested in full-cell lithium-ion batteries with a LiFePO cathode to yield a stable capacity of 350 mAh g at a rate of 0.2 A g. As electrode in K-ion batteries, CoSn@C composites presented a stable capacity of around 200 mAh g at 0.2 A g over 400 continuous cycles, and NiSn@C delivered a lower capacity of around 100 mAh g over 300 cycles.