Chemical property in heavy ion collisions

K^-/K⁺ and p̄/p ratios measured in 158 A·GeV Pb + Pb collisions are shown as a function of centrality and transverse momentum (P_t). Little significant centrality dependence in neither K^-/K⁺ nor p̄/p ratios are observed and they are almost constant as a function of P_t. The chemical freeze-out temperature T_ch and the chemical potentials for both light and strange quarks (μ_q,μ_s) are extracted by comparing the present data with simple model predictions. The μ_q,μ_s and T_ch from the NA44 are compared with those obtained from similar analysis of SPS S + A and AGS Si + A data. The chemical freeze-out temperature T_ch in GERN energy is higher than thermal freeze-out temperature T_fo which is extracted from transverse momentum distribution of charged hadrons. In AGS energy T_ch is close to T_fo.