Channel assignment mechanisms in multi-channel wireless networks are often designed without accounting for adjacent-channel interference (ACI). To prevent such interference between different users in a network, guard-bands (GBs) are needed. Introducing GBs has significant impact on spectrum efficiency. In this paper, we present channel assignment mechanisms that aim at maximizing the spectrum efficiency. More specifically, these mechanisms attempt to minimize the amount of additional GB-related spectrum that is needed to accommodate a new link. Similar to the IEEE 802.11n and the upcoming IEEE 802.11ac standards, our assignment mechanisms support channel bonding, and more generally, channel aggregation. We first consider sequential assignment (i.e., one link at a time), and we formulate the optimal ACI-aware channel assignment that maximizes the spectrum efficiency as a subset-sum problem. An exact exponential-time dynamic programming (DP) algorithm, a polynomial-time greedy heuristic, and an epsilon-approximation are presented and compared. Second, considering a set of links (batch assignment), we derive the optimal ACI-aware exponential-time assignment that maximizes the network's spectrum efficiency. The optimal batch assignment is compared with the sequential assignment. Results reveal that our proposed algorithms achieve considerable improvement in spectrum efficiency compared to previously proposed schemes. Published by Elsevier B.V.