Yes you can earn interest on cash in an Investment ISA and the profits are tax-free as long as they are within your contribution limits. Although, be careful as some providers do not pay any interest at all. You can see our comparison of the best interest rates on uninvested cash here. Some providers offer low cost stocks and shares ISAs and earn money on the interest on cash instead. Whilst others will lure new customers in with high interest on univested cash, or cash-like products, with the hope of earning fees on stocks and fund costs.