Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; Knowledge of the mean-free-path distribution of heat-carrying phonons is key to understanding phonon-mediated thermal transport. We demonstrate that thermal conductivity measurements of thin membranes spanning a wide thickness range can be used to characterize how bulk thermal conductivity is distributed over phonon mean free paths. A noncontact transient thermal grating technique was used to measure the thermal conductivity of suspended Si membranes ranging from 15–1500 nm in thickness. A decrease in the thermal conductivity from 74–13% of the bulk value is observed over this thickness range, which is attributed to diffuse phonon boundary scattering. Due to the well-defined relation between the membrane thickness and phonon mean-free-path suppression, combined with the range and accuracy of the measurements, we can reconstruct the bulk thermal conductivity accumulation vs. phonon mean free path, and compare with theoretical models. ; We acknowledge support from “Solid State Solar-Thermal Energy Conversion Centre (S3TEC),” an Energy Frontier Research Centre funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Grant No. DE-SC0001299/DE-FG02-09ER46577, the Academy of Finland under Grant No. 252598, and the EU FP7 ENERGY FET project MERGING Grant Agreement No. 309150 and the Spanish Plan Nacional project TAPHOR (MAT-2012-31392). ; Peer Reviewed