A dual-core fiber in which one of the cores is doped with germanium and the other with phosphorus is used as an in-line Mach–Zehnder dispersive interferometer. By ensuring an equal length but with different dispersion dependencies in the interferometer arms (the two cores), high-sensitivity strain and temperature sensing are achieved. Opposite sensitivities for high and low wavelength peaks were also demonstrated when strain and temperature was applied. To our knowledge this is the first time that such behavior is demonstrated using this type of in-line interferometer based on a dual-core fiber. A sensitivity of (0.102+/-0.002) nm∕με, between 0 and 800 με and (−4.2+/-0.2) nm∕°C between 47°C and 62°C is demonstrated.