Some of the Rollei 135 films are on polyester substrates, but it appears RPX400 is on triacetate (info from macodirect.de which has better technical info than anyone else I know of). This probably doesn't matter much to most of you, but it does make a difference for those using daylight tanks like Rondinax or (possibly) Lab-box. My Rondinax guillotine won't cut the polyester films, so after winding it in you have to get the tank into the dark bag, open it up and cut the film with scissors! I have a Lab-box, which may have a slightly better guillotine, but I haven't tried it.