We present the results of experimental determination of the heat capacity of the pyrochlore Er ₂ Ti ₂ O ₇ as a function of temperature (0.35–300 K) and magnetic field (up to 9 T), and for magnetically diluted solid solutions of the general formula (Er _{1− x} Y _x ) ₂ Ti ₂ O ₇ ( x ≤0.471). On either doping or increase of magnetic field, or both, the Néel temperature first shifts to lower temperature until a critical point above which there is no well-defined transition but a Schottky-like anomaly associated with the splitting of the ground state Kramers doublet. By taking into account details of the lattice contribution to the heat capacity, we accurately isolate the magnetic contribution to the heat capacity and hence to the entropy. For pure Er ₂ Ti ₂ O ₇ and for (Er _{1− x} Y _x ) ₂ Ti ₂ O ₇ , the magnetic entropy as a function of temperature evolves with two plateaus: the first at R ln ⁡ 2 , and the other at R ln ⁡ 16 . When a very high magnetic field is applied, the first plateau is washed out. The influence of dilution at low values is similar to the increase of magnetic field, as we show by examination of the critical temperature versus critical field curve in reduced terms.