The traditional polymer light-emitting device is a solid-state device in which the polymer thin film is sandwiched between two electrodes. Recently, we reported a different type of device: the polymer solution light-emitting device (SLED), in which the light-emitting medium is a thin layer of polymer solution sandwiched between two electrodes. Tn this article, we studied the light-emission mechanism of the SLEDs by cyclic voltammetry, transient electroluminescence measurement, current-light-voltage (I-L-V) measurement, and the optical microscopy observation on a surface cell SLED. The turn-on speed of the light-emission of the SLED with I tm gap was found to be ea. 40 ms, which is similar to that of the dye ECL cells. The voltage for charge injection of the SLED devices agrees with the potential difference between the onset potentials of the oxidation for radical cation formation and the reduction for radical anion formation. However, the light emission turns on at a higher voltage, and the turn-on voltage depends on the thickness of the SLED solution layer and the operation temperature. In the surface cell SLEDs, light emission occurs at the cathode side when the solution used is BDOK-PF in DCB, while it is at the anode side when the solution is MEH-PPV in cyclohexanone. For the SLEDs with the addition of dye molecules, the light emits from the luminescent molecules with lower exciton energy (lower energy gap). On the basis of the experimental results, we conclude that the device mechanism is due to electro-generated chemiluminescence.