[robinpatten]

I once marinated some steak tips. I figured longer is better. So I let them marinade for 24 hours. They were horrible. I don't remember the term for over marinading but it turned every thing to rubber.
How long is to long. I have chicken marinading now. I'm thinking 12 hrs will be ok.

[OregonYeti]

Marinating will not turn anything to rubber, as far as I know.

What are you marinating the chicken in? Usually a few hours is all that's needed, maybe 8 at most for some recipes. For smaller pieces of meat, maybe 2-3 hours max.

I think your cooking method after marinating is more likely suspect. How did you cook it?

[LollaRossa]

What did your marinade consist of? If it was anything acidic you will begin to "cook" the tips. Then to heat cook them further will make then tougher and "rubbery".

Another question. What cut of beef did the tips come from and what was their grade? These as well would affect the outcome depending on how you proceeded to cook the tips.

[DC Sunshine]

I agree with Lolla. If there was vinegar or cirtus in the marinade, you should not leave it very long, say 30 minutes only. Oil based marinades with no acid component are ok to be left overnight.

That's probably why they went like rubber.