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The spin correlations of the S = 1 Heisenberg antiferromagnetic chains in AgVP2S6 have been studied by neutron scattering at temperatures of the order of T~~\0~2J/kB.On the polycrystalline samples, with a careful choice of experimental conditions, we have observed a Haldane gap of E g =26 meV at the antiferromagnetic point at q =/r. The excitation spectra are consistent with a mode that has a spin-wave velocity (7 = 150 meV and a correlation length §«5.5, close to C/E g . These results are in quantitative agreement with the predictions and numerical results on the Haldane state.PACS numbers: 75.25. +Z, 75.10.Jm, 75.40.Gb Haldane's predictions [1,2] concerning the behavior of the isotropic-exchange, spin 5 = 1, antiferromagnetic chains triggered a continuous attraction to this apparently simple but strong statement that distinguishes between the integer and half-integer spin systems, with and without a gap for elementary excitations, respectively. The consequent numerical work as well as the results on exactly solvable models have been reviewed recently by Affleck [3]. The numerous quantum Monte Carlo calculations seem to give a consistent set of data in what concerns both the energy scale of the excitation spectrum and the dynamic correlations in the Haldane state [4][5][6][7][8].The experimental work in this fascinating field has mostly concentrated on two 5 = 1 chain compounds, CsNiCb and Ni(C 2 H8N2)2N0 2 (C104), or NENP. After the first demonstration of the existence of the Haldane gap by neutron inelastic scattering [9,10] the more detailed investigations using neutrons [11][12][13][14], as well as results on high-field magnetization [15,16], on electron paramagnetic resonance [17], and on nuclear magnetic resonance [18], for example, have pointed out the particular properties of the 5 = 1 spin chains.Another 5 = 1 chain compound with a gap for magnetic excitation is AgVP2S6, with well separated V 3+ chains embedded in a layer matrix [19]. Originally observed by magnetic susceptibility [20], the gap was confirmed in a preliminary inelastic-neutron-scattering study a while ago [21]. These results stimulated the more detailed investigation reported in the present Letter. The drawback of AgVP2S6 for neutron-scattering studies is that it can be prepared in sufficient quantity only in polycrystalline form. However, as will be shown below, the powderaveraged data obtained on the powerful time-of-flight spectrometers at Institut Laue-Langevin (ILL) and Rutherford Appleton Laboratory (RAL) give rather detailed information on the dynamic spin correlations. Especially, we are able to demonstrate that the gap mode does start from the q=K ID reciprocal point. Furthermore, applying a simple spin-wave model we can extract a value for the correlation length and for the spin-wave velocity of the excitations. All these results give a consistent picture that agrees quantitatively with the quantum Monte Carlo predictions [4][5][6][7][8].The static magnetic properties of AgVP2S6 have been examined by both magnetic-susceptibil...