This paper presents data collected by an automated system designed to measure the seasonal cycle of both the quantity and isotopic composition of soil respiration. The results support the hypothesis that deep soil respiration at the BOREAS Northern Old Black Spruce site is sensitive to soil thaw and that much of this CO2 represents decomposition of old organic matter, rather than simply root respiration. This is the first study to use a completely automated soil gas sampling system in the field to measure the soil CO2 profile and collect samples of soil air for subsequent isotopic analysis. During the summer of 1999, deep soil respiration varied linearly with 50 cm temperature, with a slope of 0.2 kg C ha-1 d-1 °C-1. Late in the season, this respiration was dominated by decomposition of old soil organic matter (fixed from the atmosphere centuries ago). This study has taken an important step toward clarifying the temperature sensitivity of deep soil CO2 production in black spruce ecosystems through direct measurements within the soil and to quantify the seasonal variation of the relative contributions of recently fixed versus old carbon in deep soil respiration.