The authors present the study of the modulation mechanisms in photoreflectance (PR) spectroscopy of an ensemble of self-assembled semiconductor quantum dots (QDs). To distinguish between possible factors contributing to the total modulation efficiency of QD transitions, a photoreflectance excitation expt. was performed on an InAs/GaAs quantum dot structure grown by solid-source MBE. The intensity of PR features related to QDs changes in a function of the wavelength of the pumping laser, tuned from above-GaAs band gap down to below wetting layer ground state transition. Based on this dependence most of the QD PR signal intensity originates from the modulation of the built-in elec. field caused by carriers photogenerated in GaAs layers. Also the modulation of QD transitions related to a possible modification of the dot properties due to filling them with carriers is negligible in PR expt. on an ensemble of dots. An addnl. confirmation of the PR results was obtained by using contactless electroreflectance (CER), demonstrating that the line shape of PR and CER QD resonances is almost identical in both spectra. Thus, the QD transitions can be analyzed by using the std. low field line shape functional form applicable in any electromodulation spectroscopy.