A double‐layer encapsulant of phosphorous glass (PSG)/Si3N4is shown to be useful for postimplantation annealing of sulfur‐implanted InP up to 900 °C without surface deterioration. Under high‐dose implantation and high‐temperature annealing conditions, however, a highly conductive layer is formed near the surface of the InP. A highly compensated orpregion is also produced in the deeper part of the implanted layer. The existence of both thermally‐induced and damage‐induced conductivity must be taken into account to estimate the electrical activity of implanted species from sheet carrier concentration. An estimated real electrical activity of sulfur implanted into InP at room temperature has been established by subtracting damage‐induced carrier concentration obtained from argon implantation data. A maximum electrical activity of 68% was obtained on a sample implanted to a dose of 1×1014S+/cm2and annealed at 900 °C for 20 min.