•Thermal stability of rocksalt and ilmenite structure HEOs was compared.•Electronic structure of ilmenite structure HEOs was investigated by UV–vis test.•Electrical conductivity of ilmenite structure HEOs was researched.•The effect of sluggish diffusion on ionic conductivity was discussed and analyzed. High entropy oxides (HEOs) are an emerging material and widely investigated due to their unique high entropy effect. Since the first report on rocksalt structure HEOs in 2015, a variety of HEOs have been synthesized successively. Previously, we found Mg28Co17Ni19Cu16Zn16O96 possessed higher single-phase formation ability and better thermal stability than Mg19.2Co19.2Ni19.2Cu19.2Zn19.2O96. However, there are relatively few studies devoted to the effect of elemental composition of Mg-Co-Ni-Cu-Zn on the phase structure of Mg-Co-Ni-Cu-Zn-metal-O. In this work, we designed, synthesized and compared the phase structure combined with optoelectrical properties of Mg19.2Co19.2Ni19.2Cu19.2Zn19.2Ti96O288 and Mg28Co17Ni19Cu16Zn16Ti96O288. Results indicate that the composition of Mg28Co17Ni19Cu16Zn16O96 which is more stable than Mg19.2Co19.2Ni19.2Cu19.2Zn19.2O96 in rocksalt structure HEOs can make Mg28Co17Ni19Cu16Zn16Ti96O288 more stable than Mg19.2Co19.2Ni19.2Cu19.2Zn19.2Ti96O288 in ilmenite structure HEOs. Notably, the attenuation of sluggish diffusion effect of Mg28Co17Ni19Cu16Zn16Ti96O288 may effectively improve the electrical conductivity by composition and phase stability optimization.