The paper investigates the characteristic features of the selective dissolution of the magnesium alloy NZ30K in the Ringer–Locke solution. As found, it is subjected to pitting corrosion. Metallographic analysis using a scanning electron microscopy reveals that the pitting surface has a developed relief characteristic of selective dissolution of steels and alloys. The characteristic features of the selective dissolution of alloy components in pitting are studied using the calculated coefficients of selective dissolution of zinc Z_Zn, zirconium Z_Zr, and neodymium Z_Nd. As believed, if these coefficients are less than one, then, the main component of the alloy, magnesium, dissolves more intensively than a specific alloying element in combination with magnesium. And, if the coefficients Z_{Zn, Zr, Nd} >1, then, it is vice versa. As found, the intensity of selective metal dissolution on the side surface of the pitting increases in the following sequence: Zr, Mg, Nd, Zn; Zr, Nd, Mg, Zn or Mg, Zr, Nd, Zn. Under such conditions, the pitting surface is enriched with zirconium or magnesium and depleted in Mg, Nd, Zn or Zr, Nd, Zn. This contributes to the solid-phase diffusion of zirconium atoms into the alloy bulk, and magnesium, neodymium, and zinc in the opposite direction in the first case, and magnesium atoms into the alloy bulk, and Zr, Nd, and Zn in the opposite direction in the second case. As shown, the processes of solid-phase diffusion of alloy components supports the anodic dissolution of metals in pitting, which is provided by cathodic reactions of oxygen reduction in their vicinity and hydrogen on their surface, which is formed as a result of hydration of corrosion products. As found, under such conditions, the magnesium content on the side surface of the NZ30K alloy decreases from 95.67 down to 71.84...82.57 wt.%, and it is covered with a passive film with an oxygen content from 10.02 up to 17.18 wt.%. As found, at the bottom of the pitting, the intensity of selective metal dissolution increases in the following sequence: Mg, Zn, Nd, Zr. Under such conditions, the surface of the pitting is enriched with magnesium and depleted in Zn, Nd, and Zr. This contributes to the solid-phase diffusion of magnesium atoms into the alloy bulk, and Zn, Nd, and Zr to the pitting surface. This supports the anodic processes, which are provided by cathodic hydrogen reduction reactions on their surface. As found, the oxygen content on the pitting surface decreases down to 1.92...7.8 wt.%. This shows that the passive film is denser on the side surface of the pitting than on its bottom. As proved, the pittings on the surface of the NZ30K alloy are repassivated, and, therefore, it can be considered as a material for the production of biodegradable implants.