Montreal in 1881 was highly segregated along four distinct social dimensions: language, religion, socioeconomic status, and sector of employment. By 1901 the population had doubled, and we examine changes in residential distributions over the two decades. Despite the increased integration of certain groups, segregation remains high, and multiple dimensions are still discernible. In addition to long-established communities of French Canadians, Irish Catholics, and Anglo-Protestants, we see new streams of immigrants occupying their own patches in the urban fabric. To make meaningful observations of sociospatial changes over two decades, we used a geographic information system (GIS) to situate individual census households with spatial precision on 1 of 12,000 lots in 1881 and 30,000 in 1901, so that we could reaggregate them into meaningful districts of different scales and districts with identical boundaries for both years of observation, thereby overcoming the major methodological problems hindering previous comparative analyses. Coupling well-established statistical indexes of segregation and diversity in a GIS framework lends new analytic power to grasp the scale of phenomena and inquire into behavioral choices of nineteenth-century households. The empirical evidence shows how both concentration and diversity were built into the urban fabric. This study also offers methodological cues for comparative studies in other places and periods.