This study examines determinants of the formalization of HRM practices with small firms. We derive five hypotheses that identify possible determinants of the level of formalization, including firm size, family business, the availability of an HRM department or HRM manager, and the existence of a formal business plan. We test these hypotheses using data on more than 700 Dutch small firms. We find that, within this sample of small firms, larger firms apply more formalized HRM practices than smaller firms do. However, once we take certain contextual variables into account, the direct relation with firm size becomes substantially less. Indirect relations with firm size also exist: firm size is a determinant of the probability that an HRM department is present, which in turn is related to the formalization of all HRM scales. Finally, family businesses apply less formal HRM practices, as do businesses without a business plan.