Potential hydrocarbon-vapor intrusion pathways into a building through a concrete slab-on-ground were investigated and quantified under a variety of environmental conditions to elucidate the potential mechanisms for indoor air contamination. Vapor discharge from the uncovered open ground soil adjacent to the building and subsequent advection into the building was unlikely due to the low soil-gas concentrations at the edge of the building as a result of aerobic biodegradation of hydrocarbon vapors. When the building’s interior was under ambient pressure, a flux of vapors into the building due to molecular diffusion of vapors through the building’s concrete slab (cyclohexane 11 and methylcyclohexane 31 mg m−2 concrete slab day−1) and short-term (up to 8 h) cyclical pressure-driven advection of vapors through an artificial crack (cyclohexane 4.2 × 103 and methylcyclohexane 1.2 × 104 mg m−2 cracks day−1) was observed. The average subslab vapor concentration under the center of the building was 25,000 μg L−1. Based on the measured building’s interior vapor concentrations and the building’s air exchange rate of 0.66 h−1, diffusion of vapors through the concrete slab was the dominant vapor intrusion pathway and cyclical pressure exchanges resulted in a near zero advective flux. When the building’s interior was under a reduced pressure (−12 Pa), advective transport through cracks or gaps in the concrete slab (cyclohexane 340 and methylcyclohexane 1100 mg m−2 cracks day−1) was the dominant vapor intrusion pathway.