We report the technique of controlled group V quantum well intermixing (QWI) in a compressively strained In0.76Ga0.24As0.85P0.15/In0.76Ga0.24As0.52P0.48 multiquantum well laser structure and its application to the fabrication of two-section tunable lasers. The blueshift of the band-gap energy was enhanced by capping the samples with films of SiO2 or low-temperature grown InP, while suppressed by a SixNy film with a refractive index of about 2.1. Spatially selective band-gap tuning was achieved by patterning the dielectric film into dot and strip arrays with different surface coverage. Time-of-flight secondary ion mass spectra showed that the enhanced blueshift was caused by the interdiffusion of group V atoms between the quantum wells and barriers. A group V interstitial interdiffusion mechanism is proposed for the sample capped with SiO2 and this is supported by the even more efficient intermixing induced by low-temperature InP, which contains a high concentration of excess phosphorus. A two-section tunable laser operating around 1.55 μm was fabricated using this QWI technology. A tuning range of about 10 nm was demonstrated by simply changing the current injected into the phase tuning section. © 2002 American Institute of Physics.