Sort of depends on how you pronounce it. "En masse" is not pronounced like "in mass" (check http://www.merriam-webster.com/audio.php?file=en_mas02&word=en%20mas

Sort of depends on how you pronounce it. "En masse" is not pronounced like "in mass" (check http://www.merriam-webster.com/audio.php?file=en_mas02&word=en%20masse&text=%5C%C3%A4%E2%81%BF(n)-%CB%88mas%2C%20-%CB%88m%C3%A4s%2C%20en-%5C http://www.merriam-webster.com/audio.php?file=en_mas02&word=...).