Summary form only given. Coherent optical transmissions (i.e. systems in which the phase is used the code information) have been extensively studied as a way to increase the performances of communication systems. In parallel to this, developments in the use of soliton propagation have provided an attractive solution to the requirements of high bit-rate, long-haul transmissions. The potential use of high capacity soliton-based coherent links depends on the robustness of the soliton phase with respect to the amplified spontaneous emission (ASE) noise introduced by the in-line erbium doped fiber amplifiers. We assess experimentally the phase jitter, and compare it to theoretical predictions, We have demonstrated the feasibility of phase shift keying on a soliton transmission system. The phase standard deviation is kept low.