Prior studies of inflation of circular membranes of ultrathin polystyrene (PS) films have evidenced a reduced glass transition temperature (Tg) and rubbery stiffening, whose origins remain unclear. Here, we describe results from inflation of rectangular, ultrathin films of the same PS material. The bubble shapes obtained from the experiment are consistent with finite element (FE) simulations. The accuracy of three approximate solutions for modulus obtained from the inflation of the thin, rectangular films was evaluated by comparison with FE analysis. The best among the three solutions was used to determine the creep compliance and rubbery stiffness of the thin films. It is found that the reduction of Tg and the rubbery stiffening for rectangular bubbles are consistent with results obtained using circular bubbles, although there is some indication that the rectangular bubbles give somewhat greater rubbery stiffening. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012