A new group of solid electrolyte-based electrochemical devices attached with a layer of auxiliary phase (Type III) are emerging as attractive sensors for the detection of oxygenic gases such as CO2, NOx and SOx. These devices can be understood as the combination of an O-2-sensitive half cell based on a conventional solid electrolyte with an oxygenic gas-sensitive half cell based on the auxiliary phase. The combinations between half cells are extended to include 3 subgroups (IIIa, IIIb and IIIc). The electrochemical connection through the hetero-junction involved is established in a straightforward manner for IIIa, while an interfacial phase is required for IIIb and IIIc. Optimization of the auxiliary phases is shown to realize excellent sensing performances in oxygenic gases. However, this group bf sensors still has many basic and application-level problems that need to be investigated.