Polypropylene windows stretched to 1-1.5 μm in thickness have been traditionally used for soft x-ray detection on gas flow proportional counters. Theoretical calculations show that significant transmission Improvement could result from a thinner window. The trade-offs to ultimate transmission have traditionally been reliability and durability. A 0.3 um window has been fabricated to show significant improvement for light element transmission, A supporting grid was used to back the film and provide additional support and durability to allow for pressure cycling during venting of the spectrometer chamber. Significant transmission improvements were observed for ultra soft x rays in the energy range below 1 KeV.