The certification of an absolute molecular mass distribution polymer Standard Reference Material, SRM 2881, is described. SRM 2881 is an n octyl initiated, proton terminated, narrow polydispersity, low mass, atactic polystyrene. The absolute molecular mass distribution (MMD) was obtained by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). The certification includes estimates of uncertainties, both Type A (random) and Type B (systematic), for each oligomer in the sample having a concentration of at least 0.18% of the total of all oligomers on the low mass tail of the MMD and 0.02% on the high mass tail. The bottle-to-bottle variation of the packaged SRM was checked by size exclusion chromatography and found to be negligible.