The certification of the absolute molecular mass distribution (MMD) of an n-octyl-initiated, proton-terminated, low mass, atactic polystyrene (PS) by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) is fully described. This polymer has been designated Standard Reference Material (SRM) 2881. The material constituting SRM 2881 was prepared commercially for the NIST Polymers Division specifically for use as a narrow-distribution polymer standard. Anticipated uses include calibration of mass spectrometers and size exclusion chromatographs (SEC) in accordance with various international standards (described below), as well as any other application where quantitative knowledge of the entire MMD is necessary. The work described here, and in related references, provides a means to create absolute molecular mass distribution standards of any low mass, narrow polydispersity polymer including proprietary materials.