In 2017, our sales increased by 23% to €737 million. We made investments in new areas resulting in new tool wins in our epitaxy and PECVD product lines. While these new tool wins strongly contributed to our sales growth in 2017, the initial costs related to the launch of the new products in epitaxy and PECVD led to a decrease in our gross margin, which dropped from 44.2% to 41.5%. Excluding this impact, our gross margin would have been relatively stable in 2017. Operating expenses remained under control and decreased as a percentage of revenue from 30% to 26%. Operating profit increased 38% with the operating margin improving to 15.3%.
During the course of 2017, we reduced our stake in ASMPT in two steps from approximately 39% to approximately 25%. In April, we sold a 4.9% stake in a partial secondary share placement for proceeds of approximately €245 million. We used the proceeds for a new €250 million share buyback program that we started in September. In November, we sold another 9.1% stake in ASMPT that generated proceeds of approximately €445 million. We announced plans to use these proceeds for share buybacks and a capital return to our shareholders.