Layered niobic acids (H(x)K(1-x)Nb(3)O(8), x = 0-1) are reported as new solid acid catalysts for the selective hydration of ethylene oxide (EO). They are prepared by simply calcinating Nb(2)O(5)-K(2)CO(3) mixture followed by an ion-exchange process in HNO(3) solution of different concentrations. The highest selectivity for monoethylene glycol (MEG) is achieved over 95% with EO conversion of >99% at x of 0.7 under H(2)O/EO ratio of 8. Combined with the results of first-principles density functional theory calculations and Hammett indicator method, it is revealed that the suitable acid strength is more crucial for MEG selectivity than acid amount. Furthermore, a self-exfoliation of layered HNb(3)O(8) is also found during EO hydration, which proves to be another important factor for its good catalytic performance by exposing the abundant acid sites among the Nb(3)O(8)(-) nanosheets. The thermal stability investigation of HNb(3)O(8) also indicates a careful selection of characterization and application way for this layered niobic acid. Crown Copyright (C) 2011 Published by Elsevier Inc. All rights reserved.