Abstract Ageing treatments at 620 K performed in ferromagnetic Co49Ni21Ga30 shape memory single crystals have been found to generate dense dispersions of nanometric γ′ precipitates in the austenitic matrix. These particles have a strong influence on the B2–L10 martensitic transformation, decreasing the overall transformation temperatures and enhancing the transformation range (difference between the temperatures for start and finish of transformation) and hysteresis. The martensitic microstructure is strongly affected as well, in such a way that the thickness of martensitic twins depends on the size and shape of particles (spherical or elongated). The results indicate that, in case of precipitation of spherical particles with diameters ∼3–5 nm, the martensitic twins can absorb the particles and the twin thickness is still considerably larger than the precipitate diameter, while growth of larger and elongated particles with a length of 10–25 nm leads to the formation of tiny twins in between the particles with thickness of ∼3–4 nm. These small martensite twins present an additional internal modulation and satellite spots in the diffraction pattern.