The composition-temperature magnetic phase diagram of ErGe 1− x Si x (0 < x < 1) has been deduced from the powder neutron diffraction investigation of the magnetic structure of several samples in the 1.5–15 K range. These compounds present an antiferromagnetic behaviour with 7.2 < T N < 11.5 K. Four magnetic phases are present: two that are commensurate with the crystal lattice (wavevectors (1/2,0,1/2) and (0,0,1/2) and two incommensurate (wavevectors (0,0, k z and ( k' x ,0, k' z ) with a slight deviation of k x , k' x and k' z from 1/2). Whatever x, an incommensurate phase appears below T N, the wavevector being (0,0, k z ) for x < 0.40 and ( k' x ,0, k' z ) for x > 0.40. For 0.17 ≥ x ≤ 0.55, a first-order transition occurs as function of the temperature between these two phases. For x ≥ 0.65, a lock-in transition takes place at T IC, leading from the wavevector ( k' x ,0, k' z ) to (1/2,0,1/2), as was already observed in ErSi. Finally, for x < 0.17 or 0.55 < x < 0.65, the wavevectors of the incommensurate phases characterized by (0,0, k z ) or ( k' x ,0, k' z ) respectively remain unchanged in the whole temperature range below T N. For x≥0.65, a small amount of a magnetic phase characterized by the wavevector (0,0, 1/2) coexists with the main phases, below a Néel temperature T' N slightly lower than T N. In all cases, the erbium magnetic moments are colinear along the orthorhombic α-axis; the arrangement of the moments in the commensurate phases is the same as in ErSi and the incommensurate orderings correspond to sine-wave amplitude modulations. A brief account on the theoretical interpretation of this phase diagram is finally given.