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Intraday variability (IDV) of the radio emission from active galactic nucleiis now known to be predominantly due to interstellar scintillation (ISS). TheMASIV (The Micro-Arcsecond Scintillation-Induced Variability) survey of 443flat spectrum sources revealed that the IDV is related to the radio fluxdensity and redshift. A study of the physical properties of these sources hasbeen severely handicapped by the absence of reliable redshift measurements formany of these objects. This paper presents 79 new redshifts and a criticalevaluation of 233 redshifts obtained from the literature. We classifyspectroscopic identifications based on emission line properties, finding that78% of the sources have broad emission lines and are mainly FSRQs. About 16%are weak lined objects, chiefly BL Lacs, and the remaining 6% are narrow lineobjects. The gross properties (redshift, spectroscopic class) of the MASIVsample are similar to those of other blazar surveys. However, the extremecompactness implied by ISS favors FSRQs and BL Lacs in the MASIV sample asthese are the most compact object classes. We confirm that the level of IDVdepends on the 5\,GHz flux density for all optical spectral types. We find thatBL Lac objects tend to be more variable than broad line quasars. The level ofISS decreases substantially above a redshift of about two. The decrease isfound to be generally consistent with ISS expected for beamed emission from ajet that is limited to a fixed maximum brightness temperature in the sourcerest frame.

The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey III. Optical Identifications and New Redshifts