Neutron diffraction has been used to investigate the critical behavior at the onset of antiferromagnetic phase transitions in a (111) oriented Pt73Fe27 film grown on an a-axis oriented sapphire (alpha-Al2O3) substrate. As in the bulk, there is an antiferromagnetic reorientation transition from the Q(1)=2pi/a(1/2,1/2,0) phase to the Q(2)=2pi/a(1/2,0,0) phase upon cooling. The temperature dependence of the integrated intensity of the (1/2,1/2,0) and the (1/2,0,0) antiferromagnetic Bragg peaks yielded the Neeel temperature of 160.25+/-0.2 K and a reorientation transition temperature of 95+/-0.2 K. The magnetization critical exponent beta is found to be 0.368+/-0.013 for the Q(1) phase and 0.37+/-0.02 for the Q(2) phase. These critical exponents are in excellent agreement with the predictions of the 3d Heisenberg universality class. A comparison of the transition temperatures and the exponents in the film and in single crystal at the same alloy composition is presented.