An apparatus to measure the electrophoretic mobility of particles in an aliquot of a sample solution comprising - a means to create a beam of coherent monochromatic light; - an optical means to divide said coherent monochromatic light beam into a sample beam that will pass through said sample, and a reference beam; - a non-conductive sample cell comprised of - a chamber to hold said sample aliquot, - electrodes, between which a voltage may be applied across said sample aliquot contained therein, and - optical windows (10, 26) in said chamber, through which said sample beam may enter said chamber and scatter from particles in said aliquot contained therein, and a non-scattered fraction of said sample beam and a fraction of said sample beam scattered from said particles in said sample aliquot may leave said sample cell; - an optical means to collect and collimate said scattered fraction of said sample beam; - means (8, 22) to modulate the optical phase of said reference beam relative to the optical phase of said sample beam; and - an optical means in free space to combine said reference beam with said collimated scattered fraction of said sample beam, forming thereby a third coherent beam; characterized in that - said optical means to collect and collimate is further provided to collect and collimate said non-scattered fraction of said sample beam, - an array of photo detector elements is provided onto which said third coherent beam is incident, with each said photo detector element producing time varying signals therefrom, wherein said non-scattered fraction of said sample beam is incident on a single photo detector element providing thereby a forward monitor and measuring the phase modulation that was impressed upon said reference beam, permitting thereby demodulation of the signals corresponding to different scattering angles, with the phase difference between the forward monitor and each photo detector element determining an accumulated optical phase and thereby the mobility of the sample; and - electronic means to process said time varying signals received from each of said photo detector elements, said electronic means being capable of demodulating the signals corresponding to said different scattering angles utilizing signals received from said forward monitor, converting said signals into digital representations, and transmitting said resultant digital signals to computer means to calculate therefrom said electrophoretic mobility. - a means to create a beam of coherent monochromatic light; - an optical means to divide said coherent monochromatic light beam into a sample beam that will pass through said sample, and a reference beam; - a non-conductive sample cell comprised of - a chamber to hold said sample aliquot, - electrodes, between which a voltage may be applied across said sample aliquot contained therein, and - optical windows (10, 26) in said chamber, through which said sample beam may enter said chamber and scatter from particles in said aliquot contained therein, and a non-scattered fraction of said sample beam and a fraction of said sample beam scattered from said particles in said sample aliquot may leave said sample cell; - a chamber to hold said sample aliquot, - electrodes, between which a voltage may be applied across said sample aliquot contained therein, and - optical windows (10, 26) in said chamber, through which said sample beam may enter said chamber and scatter from particles in said aliquot contained therein, and a non-scattered fraction of said sample beam and a fraction of said sample beam scattered from said particles in said sample aliquot may leave said sample cell; - an optical means to collect and collimate said scattered fraction of said sample beam; - means (8, 22) to modulate the optical phase of said reference beam relative to the optical phase of said sample beam; and - an optical means in free space to combine said reference beam with said collimated scattered fraction of said sample beam, forming thereby a third coherent beam; - said optical means to collect and collimate is further provided to collect and collimate said non-scattered fraction of said sample beam, - an array of photo detector elements is provided onto which said third coherent beam is incident, with each said photo detector element producing time varying signals therefrom, wherein said non-scattered fraction of said sample beam is incident on a single photo detector element providing thereby a forward monitor and measuring the phase modulation that was impressed upon said reference beam, permitting thereby demodulation of the signals corresponding to different scattering angles, with the phase difference between the forward monitor and each photo detector element determining an accumulated optical phase and thereby the mobility of the sample; and - electronic means to process said time varying signals received from each of said photo detector elements, said electronic means being capable of demodulating the signals corresponding to said different scattering angles utilizing signals received from said forward monitor, converting said signals into digital representations, and transmitting said resultant digital signals to computer means to calculate therefrom said electrophoretic mobility.