A series of unfilled acrylonitrile butadiene rubber (NBR) composites, which consists of several types of NBR with acrylonitrile (AN) monomer unit ratio (X-AN) and sulfur or dicumyl peroxide (DPO) as a vulcanizer, were exposed to 90 MPa hydrogen gas to measure the saturated hydrogen content: C.H(0) and the volume change ratio when the sample volume is the maximum: V-MAX/V-0. The aim of this paper is to reveal the dominant characteristics for C.H(0) and V-MAX/V-0 of unfilled NBR composites. Mechanical properties, X-AN, the molecular weight of the number average molecular weight between the effective crosslinks (M-ef), the molecular weight between the chemical crosslink (M-C) and the fractional free volume (FFV) were employed to examine the relationship between the characteristics and C.H(0) and V-MAX/V-0 of the NBR composites. Judging from the results of the comparisons of C.H(0) and V-MAX/V-0 with X-AN, M-ef, and FFV, it is revealed that C.H(0) and V-MAX/V-0 were dominantly correlate with FFV and MC, respectively. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.