Collaboration between two painters was a common occurrence in the Netherlands of the seventeenth century. To date, sixteen examples are known to have resulted from the partnership of the Antwerp landscape painter, Alexander Keirincx, and the Utrecht Italianate master, Cornelis van Poelenburch (see appendix). The paintings usually combine a wooded landscape by the hand of Keirincx with a few figures added by Van Poelenburch. Two of the works are signed by both artists, one by Van Poelenburch only, and most of the others by Keirincx alone.A few of these paintings have been analyzed using infrared reflectography, which revealed that Keirincx meticulously executed his underdrawing of the landscape, without any indication for the staffage. Next, Keirincx painted the landscape, sometimes deviating slightly from his preliminary underdrawing. After that, Cornelis van Poelenburch added figures and animals, here and there inserting small brush strokes to the landscape immediately around the figures.Although Van Poelenburch may have included his staffage without consulting Alexander Keirincx, an addition by Keirincx in one of his compositions, seems to indicate a closer contact between the artists. While they lived near each other in Utrecht (1632-1636) and London (1637-1641), their collaboration did not only occur during those years, as can be concluded from two dated works (1629, 1630).A unique instance of their collaboration is the Landscape with Cimon and Iphigenia (Utrecht; replica or copy in Leipzig, figs 17 and 26). IRR analysis unequivocally confirms that the landscape and the figure zone initially were painted by Van Poelenburch, after which Keirincx added the large tree at the left. A differently prepared plank was attached to the lower part, maybe especially for that purpose. The Leipzig version lacks this feature, as well as some minor adjustments to the composition, thus indicating that it was executed after the Utrecht panel.