The certification of an absolute molecular mass distribution polymer Standard Reference Material 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). A new Taylor's expansion method for signal-axis intensity calibration of polydisperse materials is described in detail. 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 0.20% of the total of all oligomers on the low mass tail of the MMD and 0.34% on the high mass tail. Type A uncertainty arising from the MALDI sample preparation was found to be the greatest contributor to the overall uncertainty of the measurement.