Laser grain-sizer instruments provide the opportunity to study the grain size distribution of sediments across a wide size range in a short time. Automatic measurements can therefore be made, on a routine basis, for a great number of samples. Oceanic studies have proved the utility of these methods in characterizing both climatic changes and changes in sediment provenances. In addition, carbonate content is estimated either directly by CaCO3 measurement, by visual observations, or by proxies such as sediment color reflectance. Nevertheless, the grain size distribution of the carbonate fraction is still a matter of speculation, and only optical observations can distinguish the nature of each carbonate fraction. Here we present the improvements on a method to study rapidly, with a high resolution, the grain size distribution of the carbonate fraction by use of a laser grain-sizer. We describe the basic methodology and apply it to an example from the Pleistocene of the Northern Atlantic Ocean.