In a recent paper, Lu et al. showed that Li[Ni0.25Co0.5Mn0.25]O-2 and Li[Ni0.375Co0.25Mn0.375]O-2 are attractive electrode materials for Li-ion cells. Most interesting, they appear to be much less reactive with electrolyte at high temperatures than LiCoO2. Since these two materials are part of the solid-solution series Li[NixCo1-2xMnx]O-2 with 0 less than or equal to x less than or equal to 1/2, which also contains LiCoO2 when x = 0, it is possible to study the reasons for the safety advantage by a careful study of electrochemical, structural, and thermal properties as a function of x. This paper reports the structural and electrochemical properties of Li[NixCo1-2xMnx]O-2 for 0 less than or equal to x less than or equal to 1/2. The materials all show good specific capacity (between 110 and 130 mAh/g between 3.0 and 4.2 V vs. Li at 40 mA/g) and very little capacity loss over 50 cycles. Careful consideration of differential capacity measurements proves that Ni2+ and Mn4+ do, indeed, substitute for Co3+, at least for x, 0.15, as was hypothesized by Lu et al. The thermal stability of the charged cathode materials improves rapidly compared to LiCoO2, then saturates as x in Li[NixCo1-2xMnx]O-2 increases. Materials with x greater than or equal to 0.075 show approximately equal thermal stability. The reason for the variation of the thermal stability with x is the subject of a companion paper.