A simple and sensitive near-null technique for multichannel nanosecond time-resolved magnetic optical rotatory dispersion measurements is introduced with a theoretical analysis describing the basis of the technique and the effects of optical imperfections, photoselection-induced linear dichroism and birefringence, and the Faraday rotation of the solvent and cell windows. This treatment identifies a potential artifact in photolyzed samples that is associated with the coupling of photoselection-induced linear dichroism with the solvent-cell Faraday rotation. Excitation geometries that minimize this problem are described. Experimental applications of the technique to the ligand rebinding reaction of myoglobin-CO after ligand photolysis are presented in the following article.