Hi Tom, Depending on the individual word and the accent of the person speaking, "th+s" can either be pronounced /θs/ (voiceless) or /ðz/ (voice). Actually pronouncing it can be tricky, and even native English speakers can have trouble pronouncing it when they first start to do it as children. Practice is the only way to do it more naturally. I hope my answer helps. Please let me know if you have any more questions =)