Yeah, sorry about not including the code, it's a bit involved, difficult to simplify, and I was in a rush. Here's the best I can do to simplify it:

Equations to solve:

eqns = {{x[Nonacosane][z, 0] == 0.04365546804923613, x[Triacontane][z, 0] == 
   0.06241482157451062, s[Nonacosane][z, 0] == 0, s[Triacontane][z, 0] == 0}, 
 {DirichletCondition[x[Nonacosane][z, t] == 0.04365546804923613, z <= 0], 
  DirichletCondition[x[Triacontane][z, t] == 0.06241482157451062, z <= 0]}, 
 {3138.4698592919644*Derivative[0, 1][s[Nonacosane]][z, t] == 
   -0.00635626711262017*(0.0028844741322105526*s[Nonacosane][z, t] - 
     x[Nonacosane][z, t]), 3138.4698592919644*Derivative[0, 1][s[Triacontane]][
     z, t] == -0.007114201335734374*
    (0.0019632116886399483*s[Triacontane][z, t] - x[Triacontane][z, t])}, 
 {3138.4698592919644*Derivative[0, 1][x[Nonacosane]][z, t] + 
    999.0059837024828*Derivative[1, 0][x[Nonacosane]][z, t] == 
   NeumannValue[0, z >= 100] + 0.00635626711262017*
     (0.0028844741322105526*s[Nonacosane][z, t] - x[Nonacosane][z, t]), 
  3138.4698592919644*Derivative[0, 1][x[Triacontane]][z, t] + 
    999.0059837024828*Derivative[1, 0][x[Triacontane]][z, t] == 
   NeumannValue[0, z >= 100] + 0.007114201335734374*
     (0.0019632116886399483*s[Triacontane][z, t] - x[Triacontane][z, t])}}

The dependent variables

depVars = {x[Nonacosane][z, t], s[Nonacosane][z, t], x[Triacontane][z, t], 
 s[Triacontane][z, t]}

NDSolve[eqns,depVars,{z,0,1000},{t,0,5000}]

In these equations, a reaction is taking place between two states "s" and "x". It solves without a problem.

I want to calculate the accumulated rates by saying D[m[z,t],t]=Sum(rates[z,t]). I do that by adding "D[sArea[z,t],t]=Sum(rates)" and "sArea[z,0]=0" to the equations

{-9.27269625879322*^-9*s[Nonacosane][z, t] - 7.574037191466946*^-9*
    s[Triacontane][z, t] + 3.214692118485727*^-6*x[Nonacosane][z, t] + 
   3.857982934440454*^-6*x[Triacontane][z, t] == Derivative[0, 1][sArea][z, t], 
 sArea[z, 0] == 0}

This fails. I tried a completely random equation that solves in NDSolve but when added to the equation list, it fails.

Thanks, but this is a copy-paste of some code. I added “eons” (should have been “eqns”) for this post. Did you happen to try the code? It fails for me

Update: original typo fixed

That's right, the first set of equations solves fine. My question is when adding another equation (given in post) it fails. I'm adding to the notebook and will send it back in a few minutes