Hi guys,
I've posted the same message under a different subject title. This is only to present a clearer perspective of my dilemma:

I'm an undergraduate (Bachelors) applying to universities in the US for a graduate program in Chemical Engineering. I am interested in research and would want to do a PhD some day. Most of these universities have the option of opting for their PhD program at this juncture;
My worry is this: If I choose to apply to their PhD program, I would have to work through the Masters. However, if something goes wrong or if I find something unsuitable, would it be possible for me to change the university after my masters or quit with a terminal masters degree?
Thanx!

I think most science-based PhD programmes in the US are based on MS+PhD. I know that some universities will only allow you to sign up for the full 5 year PhD even though the first 2 years are spent on the MS. Also there is a lot of lab rotation during the MS years, which gives you a lot of time to work out which lab you want to work in.

I have known students in the US who on starting their PhD projects were unhappy in their choice of PhD lab but were allowed by the dept to change supervisors and project. So no fear, its never too late to change your mind.