This study measures the neutralizing capacity of bottom ash from municipal solid waste incineration ofdifferent particle sizes. We examine the effect of particle size on the weathering process, a method popularly used forstabilization of heavy metals in incineration of bottom ash. The distribution of particle sizes in municipal solid wasteincineration bottom ash is rather broad, ranging from fine powder to as large as 40mm in diameter. Although considereda by-product highly suitable as a road construction material, the high level of heavy metal leaching is an obstacle toits reuse. Weathering, a method used to reduce heavy metal leaching, is a lengthy process taking over thee months tocomplete. The chief reaction involved in weathering is carbonation occurring between Ca(OH)2 in bottom ash and CO2(g)in the atmosphere. During this process, CaCO3 is produced, causing the pH level to drop from over 12 to about 8.2and reducing heavy metal leaching. In this paper, we attempt to determine the particle size best suited for reducingthe period required for weathering bottom ash by identifying characteristics of different particle sizes that affect heavymetal leaching and neutralization.