Sea-ice crystal structure and stable oxygen isotopic composition (δ¹⁸O) were studied in the Baltic Sea. Landfast ice was studied in the Gulf of Finland with annual sampling from the same location between 1999 and 2009. Pack-ice studies were carried out in the Bay of Bothnia with annual campaigns between 2006 and 2009. In the landfast sea ice, annual maximum ice thickness was 22–60 cm of which an upper granular ice layer constituted 4–38%. Superimposed ice formation was recurring on years when granular ice contributed over 18% of total ice mass and superimposed ice contribution to the total ice mass was 0–22%. In years when no superimposed ice formation was observed, snow-ice formation contributed all of the granular ice. The contribution of meteoric ice to the total ice had large interannual variability and varied from 2 to 30% (by mass). Contribution of meteoric ice in the pack-ice samples varied between 0 and 13% (by mass) and granular ice contributed 0–70% of total ice thickness. Superimposed ice was present in only two out of 15 pack-ice samples and snow-ice formation was a more important origin of granular ice. In both areas, columnar ice δ¹⁸O values were consistent within each area and showed very small variation between years. Increases of columnar ice δ¹⁸O values along the distance from granular–columnar ice interface due to the decrease in ice growth rates were visible in all samples and pronouncedly so in the pack-ice samples.