The rush of hybrid note issuance may be about to come to an end, following a decision by Standard & Poor's to review its criteria for assigning equity credit to hybrid notes issued by corporates.S&P's global review will focus on the criteria for assigning "high" equity content to corporate hybrid capital instruments. This is particularly relevant for Australia, and for AGL Energy, Origin Energy, Santos and Tabcorp, who all received 100 per cent equity credit for their recent hybrid issues. One hundred per cent equity credit means the hybrid instrument is treated as equity and not debt for the purposes of the rating agency's calculation of debt service ratios. In other words, the issuer may be able to maintain a higher credit rating than would be allowed if the hybrid instrument was treated as debt.There was some speculation after the announcement was made that the equity credit granted to these issues would be grandfathered, but the announcement made by S&P makes it clear that any change to the criteria that results from the review will be applied to existing hybrid issues, and any change from "high" equity content to "intermediate" or "minimal" equity content could impact on the issuer's senior credit rating.One outcome of this change may be that a number of the hybrid notes issued over the last 12 months will be called much earlier than investors had anticipated. The corporate hybrid notes sold over the last year typically have very long terms to maturity, ranging from 25 years to 60 years, but they have call options that allow the notes to be redeemed after just five or six years. Investors were assured by their advisers or brokers that these call options would be exercised because the equity credit granted by S&P would fall away at this point. Thus, S&P was granting equity credit to the hybrids because it expected the call option would not be exercised, but investors were buying the hybrids because they expected the call option would be exercised. S&P is expected to bring its criteria for assigning equity content to corporate hybrid capital into line with its criteria for banks. S&P does not grant "high" equity content to hybrid notes with call options issued by banks because the reputational risk to a bank of not exercising the call option is recognised.  If S&P makes this decision it will turn the cheap "equity" raised by AGL Energy, Origin Energy, Santos and Tabcorp into expensive debt with a metaphorical stroke of a pen. The chief financial officers and corporate treasurers of these companies will then have a decision to make. The hybrid notes can be called early, under the Capital Event trigger included in the terms and conditions of the issues, or, if the notes have also received equity credit from Fitch Ratings and/or Moody's Investors Service, they can be left in place. But investors beware, if the notes are left in place without the benefit of high equity credit from S&P, there is even less incentive