Following the Haldane conjecture, the antiferromagnetic (AF) Heisenberg chain of integer spins has a singlet ground state separated from the excited states by an energy gap. Recent numerical calculations on finite AF chains with S=1 supported this conjecture and provided an approximate value for the energy gap:E G ≂0.4‖ J‖, where J is the intrachain exchange interaction. We report experimental studies on two Ni (II) quasi‐one‐dimensional (1D) AF with large intrachain interaction, J/k≂−50 K, Ni(C2H8N2)2NO2(ClO4) (NENP) and Ni(C3H1 0N2)2NO2(ClO4), (NINO). In both compounds, the magnetic susceptibility along the three crystal axes steeply decreases below T≂15 K and no 3D long‐range magnetic order could be detected down to 1.2 K. These features are consistent with the Haldane conjecture. Inelastic neutron scattering experiments performed on NENP show two energy gaps, with an average value of about 0.4‖ J‖, which are explained by a splitting of the Haldane gap by single‐ion anisotropy.