The precipitates responsible for the age-hardening effect in dilute Cu-Ni-Si alloys based on Cu2NiSi have been studied using transmission electron microscopy. The work has confirmed the findings of an earlier study by Teplitskiy et al., which showed that the precipitates formed as discs on {110} planes and indicated that they had a structure which corresponded to that of delta-Ni2Si. The orientation relationship of the precipitate and matrix found in the present work differs from that found in the earlier work, and a high degree of coherency across the precipitate matrix interface has been shown to exist. In establishing that the precipitate is delta-Ni2Si, EDX analysis has been employed and ring patterns from extracted precipitates have also been analysed. Absences from such ring patterns which were noted in the earlier work and which were thought to cast doubt on the precipitate identification have been accounted for in terms of preferred orientation effects. At peak strength the deformation of the material is believed to involve Orowan looping around the precipitates.