The bandfilling and gain behavior of thin (Lz∼400 A˚) In1−xGaxP1−zAsz(x∼0.13,z∼0.29) layers imbedded in InPp‐njunctions, or on one side of the active region of regular quaternary double heterojunctions (x∼0.09,z∼0.20), are described. Bandfilling and spontaneous emission are observed from the quaternary quantum‐well band edge to the band edge of the InP confining layers (&Dgr;Eg∼245 meV, 77 K). Laser operation on confined‐particle transitions can be observed in the range 0<h&ngr;−Eg(InGaPAs) ≲ (2/3) &Dgr;Eg. The reduced gain of a quantum‐well heterostructure is demonstrated by comparison with the behavior of conventional quaternary double heterojunctions (active layer ≳0.1 &mgr;m) grown from the same set of LPE melts. To operate as lasers quantum‐well heterostructures are shown to require greater diode lengths and higher excitation currents, consistent with large bandfilling (in a thin layer) leading to a large spectral spread in the recombination radiation. These effects are demonstrated also on quaternary double heterojunctions of standard active‐layer thickness (≳0.1 &mgr;m) but modified with the inclusion of a quantum well on one side of the active region.