[en] The crystal and magnetic structures of Y_n+1Co_3n+5B_2n (n=2, 3 and ∞) have been studied by high-resolution powder neutron diffraction. The results are compared to earlier measurements on YCo₅ and YCo₄B. A change in the regular stacking of the boron-containing plane along the c axis has been observed in the Y₂Co₇B₃ sample. Very short Co-B distances are observed, indicating that strong bonds are formed between cobalt and boron. The YCo₃B₂ compound is paramagnetic down to 2 K. The magnetic structures of Y₃Co₁₁B₄ and Y₂Co₇B₃ confirm the large variety of cobalt magnetic moments obtained in these compounds. The magnetic behaviour of the Co(2c) atoms is not significantly affected by the substitution of boron for cobalt. Cobalt atoms with significantly reduced magnetic moments are found on the 3g and 6i₂ sites in both Y₃Co₁₁B₄ and Y₂Co₇B₃. A relationship between the magnitude of the Co magnetic moment and the presence of boron in the neighbourhood of the cobalt atoms is proposed. The hybridization of the cobalt 3d electronic state with the boron 2p state is found to play a major role in the determination of the magnitude of the Co magnetic moment in the Y_n+1Co_3n+5B_2n compounds. (author)