Ilfosol-S has acquired a reputation for suddenly going bad.
According to the Ilfosol-S Material Safety Data Sheets (MSDS),
Ilfosol-S contains two developing agents, Hydroquinone and Sodium Ascorbate:
Sodium Ascorbate is the sodium salt of Ascorbic Acid (aka vitamin C)
Ilfosol-S also contains:
Sodium Sulfite
Sodium Carbonate
Water
According to Ryuji Suzuki, in order to insure a stable long life water based developer, Ascorbate should be protected with Salycilic Acid to prevent deterioration. See Suzuki's DS-10 and DS-12 developers, for example
Another approach that works very well is to dissolve the developing reagents in Triethanolamine (i.e., no water in the developer concentrate). Pat Gainer's PC-TEA is a good example.
Pat Gainer has also successfully combined sodium ascorbate with Rodinal.