I get the same thing sometimes. For some reason, the network_domain found in your sqlnet.ora file sometimes does not get appended to the tnsnames entry in which you will need to add it to your tnsnames entry. I found this behaviour particularly quirky under Linux. You may also want to check how the listener was set up for either instance as well. Does one instance have the Oracle 8 Release 8.0 compatible identification set and the other does not? Some thing to check.

In terms of the 8i setting when creating a new entry - you do not "necessarily" see it in the tnsnames.ora file until you dig deep into it. For example, SID is an Oracle 7 and 8 construct (in the Connect_data portion of the tnsnames.ora file) whereas SERVICE_NAME is an Oracle8i and above setting. This allows you to add things like INSTANCE_NAME (when in an OPS and RAC enviroment), and what not.

HOH