We used a sequential density and particle-size fractionation to evaluate where carbon (C) storage occurs within the matrix of soils under no-till in eastern Canada. Soil cores from four long-term tillage studies were collected and divided into six profile depth increments. Four soil organic matter (OM) fractions were isolated from soil using a sequential density and particle-size fractionation method; these included free light fraction (LF) OM, protected LF OM (i.e., located within aggregates), particulate OM (i.e., that associated with sand-size particles) and mineral-associated OM (i.e., that associated with silt/clay–size particles). The mineral-associated OM usually contained the largest portion of the soil organic C pool. We hypothesized that most of the increase in C storage under no-till would occur in the first three fractions. These labile fractions contained 10-20% of the total soil C and the quantity in each varied with depth. Substantial amounts of C were protected within aggregates. More of the C was present as free LF in no-till than in tilled soils. The relatively large proportion of protected OM in the PEI soil suggests that aggregation is an important mechanism for stabilization of OM in sandy soils. A greater proportion of C was located in the protected OM fraction in the tilled than in no-till soils. This suggests that plowing, by incorporating and mixing residues with soil minerals, enhances formation of aggregates and thereby protects some labile OM from mineralization. Even though there were no consistent effects of particulate OM with depth, this fraction also appears to contain potentially labile OM. We conclude that the three isolated fractions constitute a significant labile pool which would be the site of accumulation (or loss) of C.