My grandfather disappeared in Berlin in 1944/5. He was working for IG Farben and was taken away whilst at work one day. His name is Paul Diedrich. I am going to Berlin this week. How can I track his death down?
IG Farben no longer exist. There is lots of documentation about IG Farben. IG Farben is not only associated with Zyklon B, but also with one concentration camp in particular, the Buna/Monowitz concentration camp. There is a Wollheim Memorial at the former IG Farben building, named after a survivor of this camp.
However, to find out about the fate of an individual, you'll want to contact the International Tracing Service in Bad Arolsen, which has been open to the general public since 2007 Nov 28. Sorry to say that Bad Arolsen isn't very close to Berlin. It is possible to visit the ITS in person, and if you do so as a survivor or relative, you may look into original documents, but you need to file an inquiry and schedule an appointment to do so.
You can also file a request for information online, to have photocopies sent to you. I did so when they opened up in 2007. They were swamped, and the reply took a few months. You will get a faster reply, but if you hope to visit, you should visit their site and file your inquiry ASAP.