The quasi-simultaneous arrival (QSA) effect has been tested with the Carneca NanoSIMS 50. The capability of obtaining rather large values of K (ratio of secondary ions ejected per primary ions) has been used to follow the evolution of QSA over a large range of K value by changing the collection efficiency. The amplitude of the effect associated with QSA has been determined and compared to the theoretical value by using silicon and sulfur isotopic ratio measurements. QSA influence on pulse height distribution (PHD) has also been demonstrated. (C) 2004 Elsevier B.V. All rights reserved.