An extensive analysis of photometric and spectrophotometric measurements of Mars is performed. Included are details of the Martian opposition effect reported earlier by O'Leary and their relationship with corresponding measurements of various samples in the laboratory. It is found that the ferric oxides—limonite, goethite, and hematite—all show a greater opposition effect than does Mars. These samples showed very small limb-darkening, again in disagreement with the Martian case. There is finally the 0.87-μ ferric oxide band which is missing in the Martian spectrum. These three points argue strongly against limonites, goethites, or hematites as the predominant covering for the Martian surface. This interpretation does not contradict previous polarimetric data, as the polarimetric results are sufficiently uncertain as to encompass many possible substances. There remains little evidence for the presence of ferric oxides on the Martian surface besides the characteristic red color of the bright areas: this could be due to small quantities of, say, hematite. The probable thermodynamic instability of goethite under Martian conditions argues against its presence on the surface. On the positive side, the 1.05-μ minimum in the Martian spectrum reported by Tull might be due to small quantities of siderite or larger abundances of ultrabasic silicates.