Use official titles in Acknowledgements I am writing the Acknowledgements of my thesis and I would like to know if it's necessary to use the official academic titles.
For instance "Thanks Dr.X for his invaluable help in ....", but it could happen that actually Dr.X is a Professor, or he prefers the D.Phil or PhD. 
Should I make a little research to be sure that no one will feel "offended"?
Or is there some consensus about this matter?

Thanks in advance.
Yes, it is probably best to be a bit formal in acknowledgements. The difference would be two or more true peers, in which case they may collectively decide to be informal. But if you are a student, then it is wise to err on the formal side.