The increase of atmospheric CO2 concentrations has stimulated research activity at natural CO2 springs, i.e. CO2-emitting vents mostly occurring at sites of former volcanic action. Besides a number of valuable benefits (long-term CO2 enrichment; CO2 gradients over space; natural conditions; free CO2; cheap experiments; possibility to introduce selected vegetation; source of plant material for controlled-condition experiments), emission from the vents may induce variability in atmospheric (composition, temperature, vertical CO2 gradients, short-term CO2 fluctuations) and soil conditions (pH, temperature, CO2 concentrations), and create an environment differing from the CO2-enrichment scenarios. Biological investigations at CO2 springs should previously record all relevant environmental factors and their co-variance. Here we review pros and cons of CO2 springs with the aim to help the selection of the best CO2 springs and control sites to investigate plant responses to CO2 enrichment in natural conditions.