Abstract In the IRI model foF2 ‘saturates’ when R12 - the 12 months running mean sunspot numbers - is equal to 150. This is surprising since with increase in sunspot numbers, solar EUV flux would increase, which would enhance the ion production rate. Consequently, foF2 should increase if dynamic effects (i.e, ion drift velocities) do not change significantly with R12. To examine this discrepancy, we analyze ionospheric data from three Indian stations, namely Kodaikanal (10.2°N,77.5°E), Ahmedabad (23.0°N,72.6°E) and Delhi (28.6°N,77.2°E) for 35 years during the solar cycles 19,20 and 21. We find that foF2 does increase with solar activity, though the increase is faster at lower levels and slower at higher levels. We fit 2nd degree polynomials to the foF2/R12 points for each hour of the day for each month and then determine from these polynomial fits the value of R12 at which foF2 saturates. It is seen that foF2 saturates for R12 well above 150 in a very large number of cases, though a few exceptions are also there.