The performance of a newly-developed portable gas analyzer, capable of real-time measurement of CO2, SO2 and H2S concentrations in volcanic gases, was tested at La Fossa Crater, Vulcano Island. The gas analyzer was used to acquire about 3000 determinations over the fumarolic field, allowing the definition of its chemical structure and heterogeneity. Our high-resolution analysis reveals that, in December 2004, the La Fossa fumarolic field was characterized by an oxidized inner core (SO2/H2S ratios of ~3), and by more reducing conditions on its northern edge (SO2/H2S ratios of ~1; range: 0.2--3.3). CO2/(SO2+H2S) molar ratios averaged 35 ¿ 21, with overlapping compositions for rim and inner crater fumaroles. S-poor compositions (CO2/(SO2+H2S) ≥ 50) characterized the field margins, probably due to deposition of native sulfur. Based on the above data and an SO2 flux of 18 ¿ 3 t¿d-1, we estimate CO2 and H2S output rates from the volcano of 420 ¿ 250 and 4 ¿ 2 t¿d-1, respectively.