Indoor formaldehyde removal requires use of chemi-sorbents or catalysts in order to achieve meaningful removal efficiency. The objectives of this study were to determine the formaldehyde removal capacities of different sorbents, to investigate the effects of concentration and relative humidity on the capacity and to improve the understanding of the removal mechanism as related to the micropore structure, water-solubility, and competition between formaldehyde and water vapor for the adsorption and reaction sites. Five different types of media were tested for their removal capacities. Although two of the media have totally different micropore structure - one with large surface area and small pore size and another with small surface area and large pore size - they both presented large adsorption capacity, which indicated that the performance of chemi-sorbents is much more attributed to the chemical composition than the micropore structure. The effects of concentration and relative humidity were determined for the two media by using single pass breakthrough test method. The relative humidity had clear effect on the formaldehyde removal capacity of activated carbon-based media, but the effect for the activated alumina based media is not significant. The reason was elucidated by analyzing the combined effect of micropore size, water vapor capillary condensation and chemical reaction. (C) 2010 Elsevier B.V. All rights reserved.