With highly electropositive metals, such as aluminium, these selenides are prone to hydrolysis:

  
    
      
        
          
            Al
            
              2
            
            
              
            
          
          
            Se
            
              3
            
            
              
            
          
          +
          3
          
          
            H
            
              2
            
            
              
            
          
          O
          ⟶
          
            Al
            
              2
            
            
              
            
          
          
            O
            
              3
            
            
              
            
          
          +
          3
          
          
            H
            
              2
            
            
              
            
          
          Se
        
      
    
    {\displaystyle {\ce {Al2Se3 + 3 H2O -> Al2O3 + 3 H2Se}}}
  Alkali metal selenides react with selenium to form polyselenides, Se2−n, which exist as chains.
Its use as a photoconductor in plain-paper copiers once was a leading application, but in the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers switched to organic photoconductors. Though once widely used, selenium rectifiers have mostly been replaced (or are being replaced) by silicon-based devices. The most notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal-oxide varistors. Zinc selenide was the first material for blue LEDs, but gallium nitride dominates that market. Cadmium selenide was an important component in quantum dots. Sheets of amorphous selenium convert X-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel X-ray cameras. Ionized selenium (Se+24) is one of the active mediums used in X-ray lasers.Selenium is a catalyst in some chemical reactions, but it is not widely used because of issues with toxicity. In X-ray crystallography, incorporation of one or more selenium atoms in place of sulfur helps with multiple-wavelength anomalous dispersion and single wavelength anomalous dispersion phasing.Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers. Selenium intensifies and extends the tonal range of black-and-white photographic images and improves the permanence of prints.75Se is used as a gamma source in industrial radiography. Pollution
In high concentrations, selenium acts as an environmental contaminant. Sources of pollution include waste materials from certain mining, agricultural, petrochemical, and industrial manufacturing operations.