The lithium insertion reactions with tin (II) oxide (SnO) and tin-based composite oxide (abbreviated as TBCO) are studied by solid ⁷Li-NMR Knight shift, T₁ and T_1ρ relaxation rate, TEM and in situ XRD methods. By the insertion reaction for SnO, the lithium oxide and β-tin are produced first at Li/Sn = 2; at Li/Sn = 3 to 6 the products are not simple and a mixture of LiSn₂, LiSn, Li₅Sn₂ and Li₇Sn₂ alloys is detected during the insertion. For the TBCO, which is revealed as amorphous, mainly constituted by randomly distributed very short-range (order of 10^-9 m) regions by TEM observation, it is found that electrochemically inserted lithium forms Li₂O and produces metallic tin (Sn) in the first step (Li/Sn <2), then the highly ionic lithium-tin alloys, Li₇Sn₂ (and Li₇Si₂), are produced in the second step (Li/Sn >2). During the second step, the Li/Sn ratio of formed lithium-tin alloy is kept at almost 4. By the analyses of ⁷Li NMR Knight shifts, line shape and in situ XRD, the lithium-inserted TBCOs are characterized as almost amorphous and mixtures of highly ionic components.