A study of B 0 → η c ϕ and B 0 → η c π + π − decays is performed using pp collision data corresponding to an integrated luminosity of 3.0 fb−1, collected with the LHCb detector in Run 1 of the LHC. The observation of the decay B 0 → η c ϕ is reported, where the η c meson is reconstructed in the $$ p\overline{p} $$ , K + K − π + π −, π + π − π + π − and K + K − K + K − decay modes and the ϕ(1020) in the K + K − decay mode. The decay B 0 → J/ψϕ is used as a normalisation channel. Evidence is also reported for the decay B 0 → η c π + π −, where the η c meson is reconstructed in the $$ p\overline{p} $$ decay mode, using the decay B 0 → J/ψπ + π − as a normalisation channel. The measured branching fractions are $$ \begin{array}{l}\mathrm{\mathcal{B}}\left({B}_s^0\to {\eta}_c\phi \right)=\left(5.01\pm 0.53\pm 0.27\pm 0.63\right)\times {10}^{-4},\hfill \\ {}\mathrm{\mathcal{B}}\left({B}_s^0\to {\eta}_c{\pi}^{+}{\pi}^{-}\right)=\left(1.76\pm 0.59\pm 0.12\pm 0.29\right)\times {10}^{-4},\hfill \end{array} $$ where in each case the first uncertainty is statistical, the second systematic and the third uncertainty is due to the limited knowledge of the external branching fractions.